What Is a Shipper's Letter of Instruction (SLI), and How Does It Differ from a Shipping Instruction

Exporters dealing with a freight forwarder for the first time often run into two documents with confusingly similar names — the Shipper's Letter of Instruction and the shipping instruction — and it's easy to assume they're just two names for the same thing. They're related, and they do overlap in purpose, but they play distinct roles at different points in the export process, and mixing them up can cause real friction when a forwarder is waiting for the wrong document.

This guide lays out what each one actually covers, why they exist as separate documents rather than one combined form, and how exporters typically decide whether they need one, the other, or both.

What an SLI Covers

A Shipper's Letter of Instruction is, at its core, an authorization document — it's how an exporter formally tells a freight forwarder or customs broker "you're acting on my behalf for this export activity," along with the standing information the forwarder needs to do that job. That typically includes the shipper's legal business details, authorization for the forwarder to act as forwarding agent and prepare export documentation on the shipper's behalf, and information the forwarder needs to classify products correctly for export purposes.

Because the SLI is often framed around the relationship or arrangement rather than a single shipment, it can include standing preferences that apply broadly — general routing preferences, insurance arrangements, or packing instructions that the exporter wants applied consistently rather than re-specified every time. Some exporters treat the SLI as a document set up once with a forwarder and updated only when something material changes, rather than something prepared fresh for every shipment.

What a Shipping Instruction Covers

A shipping instruction, by contrast, is squarely focused on one thing: telling the carrier or forwarder exactly what should appear on the bill of lading for a specific shipment. That means precise wording for the shipper name and address, the consignee, the notify party, shipping marks, container and seal numbers where relevant, and a description of the cargo — the exact fields that will end up printed on the bill of lading itself.

Because the bill of lading is a document of title with legal consequences for who can claim the cargo at destination, precision on the shipping instruction matters in a very direct way — an error here doesn't just create paperwork friction, it can carry straight through onto a legal document that a bank, consignee, or customs authority may later rely on. This is why shipping instructions are typically submitted per shipment, close to the booking confirmation, rather than treated as a standing document the way an SLI often is.

Why Two Separate Documents Exist

The practical reason these two documents stay separate, rather than being merged into a single form, comes down to how differently they're used and how often each needs to change. An SLI's content — company details, agent authorization, standing preferences — genuinely doesn't change from shipment to shipment for most exporters, so re-collecting it every time would be redundant. A shipping instruction's content, by contrast, is specific to the actual goods, buyer, and routing of one particular shipment, and needs to be prepared fresh (or at least reviewed and confirmed) every single time.

Keeping them separate also reflects who tends to be involved in preparing each. The SLI is often something set up once by whoever manages the exporter's relationship with the forwarder — sometimes a logistics or compliance role — while the shipping instruction is frequently prepared by whoever is handling the specific order, such as a sales or export documentation team member working from that shipment's purchase order and packing details.

Where Each Document Fits in the Export Timeline

Sequenced against the export process, the SLI typically comes first — often before there's even a specific shipment in mind, as part of establishing the forwarder relationship or updating standing details. The shipping instruction comes much later, once a specific order is being booked: after the goods are ready, the booking is confirmed with the carrier, and the exporter has the exact shipment details — cargo description, quantities, marks — needed to specify what should appear on the bill of lading.

In practice, some forwarder relationships fold elements of both into a single working process, especially with smaller or occasional exporters, where a forwarder might collect standing SLI-type information once and then simply ask for shipment-specific details by email or a booking form for each shipment rather than issuing two formally separate documents every time. The underlying distinction between authorization/standing-information and shipment-specific bill-of-lading data still holds, even when the paperwork itself is streamlined.

Deciding What You Actually Need to Prepare

For a first-time exporter, the most useful starting question is simply what stage of the relationship they're at with a given forwarder. If it's the first time working with a particular forwarder, an SLI-type conversation is usually the right first step — establishing authorization and sharing the standing company and product information the forwarder will need going forward. Once that relationship exists, attention shifts to getting the shipping instruction right for each individual shipment, since that's the document most directly tied to what ends up on the legal shipping document.

Asking the forwarder directly which of the two they need, and in what format, is a reasonable and common step — different forwarders have different intake processes, and some use their own booking portal or form rather than a traditional standalone SLI or shipping instruction document. What matters more than the exact document name is understanding which category of information — standing authorization and company details, versus shipment-specific bill-of-lading data — is actually being requested, so the right people within the exporting business can supply accurate information rather than guessing at what's needed.

Common Fields Requested on a Shipping Instruction

While the exact template varies between forwarders and carriers, a shipping instruction generally asks for the same core categories of information, since these are what the bill of lading ultimately needs. That includes the shipper's exact legal name and address as it should appear, the consignee's name and address (or a placeholder such as "to order" if the shipment is structured that way for payment or title purposes), and any notify party who should be alerted when the cargo arrives, which is often but not always the same as the consignee. It also includes shipping marks and numbers used to identify individual packages, and a cargo description that matches what's on the commercial invoice and packing list closely enough to avoid discrepancies.

Getting each of these fields right matters differently depending on the field. A minor formatting inconsistency in the shipper's address is usually a low-stakes issue; an incorrect consignee name, on the other hand, can affect who is legally entitled to claim the cargo at destination, which is why it's worth having someone double-check the consignee and notify party details specifically before the shipping instruction is submitted, rather than treating the whole form as equally low-risk to fill in quickly.

Keeping SLI Information Current Over Time

Because an SLI is often set up once and relied on across many shipments, it's worth treating it as a living document rather than something filed away and forgotten. Company details can change — a new registered address, a change in authorized signatories, or a shift in which products a business regularly exports — and an SLI that's gone stale can create friction later, particularly if the forwarder is relying on outdated information when preparing export documentation on the exporter's behalf.

A reasonable practice is to review the SLI on file with a primary forwarder periodically, or trigger a review whenever something material changes in the business, rather than waiting for a problem to surface before updating it. This is a low-effort habit compared to the cost of a documentation error traced back to outdated standing information, and it keeps the relationship between exporter and forwarder running on accurate assumptions rather than whatever was true when the SLI was first drawn up.