ShipStation can automatically create customs forms for your international shipments. We can print two forms, PS Form 2976 and 2976A (you can find more information about them here).
Head to your Account Settings (the gear icon in the upper right).
Next, select Shipping and then International Settings from the sidebar on the left.
Here you'll have the ability to choose a default content type for your international shipments:
You can then choose what happens to the shipments in a non-delivery situation, and then how your items are declared in the form.
There are two settings that will auto-fill your customs forms for your shipped items: "Create declarations from order items," and "Use pre-defined values." The third option will require you to input your own customs information in the order. The first will auto-fill what information our system has for each of the products, while the second will put in whatever information you specify here.
Finally, you can choose the name with which you sign the customs forms when they are created. If you are submitting the customs information electronically via UPS or FedEx, you will want to make sure that you have a signature, though it is otherwise optional.
When you've finished setting up your forms, click Save Changes, and your customs forms will have the settings you specified.