USPS international shipments use integrated labels if a separate customs form is not required. Integrated labels display the customs information directly on a single label. These labels are transmitted to ShipStation from the postage provider already formatted with the fields and information required by the carrier, and there is no need to print additional forms for these packages.
Basically, you'll see two types of integrated customs labels:
A single-part label (a.k.a. "single ply")
A three-part label (1 - Proof of Payment, 2 - Customs Declaration, 3 - Dispatch Note)
The single-part label is easy to use - it's like any other shipping label.
The three-part label requires you to put the labels, in order, inside a USPS-provided window pocket. These window pockets are free and can be ordered from your local post office or from the USPS website.
ShipStation doesn't control which international shipments get the single-part labels and which get the three-part labels. The USPS postage providers determines this according to USPS requirements (e.g., declared value and destination country), which are subject to change.
For example, USPS recently decided that shipments to Australia, Canada, Costa Rica, and US Military destinations no longer require three-part labels (see Appendix D of this USPS document).
For more information on shipping internationally, review the International Shipping section of our help guide.