A domestic packing slip and one crossing an international border look similar at a glance, but the international version usually carries a bit more responsibility, since customs officials may also review it.
A clear description of contents matters more
Customs officials rely partly on the accompanying documentation to assess a shipment — vague descriptions ("miscellaneous goods") can cause delays, while specific, accurate item descriptions move through customs more smoothly.
Country of origin
Many customs processes want to know where an item was actually manufactured, not just shipped from — this is often a separate detail from the sender's address and matters for duty calculations.
A declared value, separate from any invoice
Even though a packing slip doesn't show retail pricing, international shipments often need a stated customs value for duty assessment — this is typically a specific customs declaration, not the commercial sale price, and is often provided on a separate customs form (like a CN22/CN23) rather than the packing slip itself.
HS codes, where applicable
Harmonized System codes classify goods for customs purposes and are sometimes required on accompanying paperwork for certain product categories or destinations — worth checking whether your products need one for the countries you ship to.
Keep the packing slip and customs form clearly separate
The packing slip still serves its original purpose — confirming contents for the recipient — while customs-specific documentation handles the regulatory side. Don't try to merge the two into one overloaded document.