To automatically set customs settings for international orders, head to your Account Settings (the wrench icon in the upper right).
Next, select Shipping and then International Settings from the sidebar on the left.
From here you can automatically set new default values for Contents Type, Non-Delivery Option, and Customs Declarations. To automatically sign your customs forms, enter the name of the person who handles international shipments in your company into the Sign Customs As field. This should be a valid name and not a company name.
Please note that Stamps.com, Express 1, and FedEx are only able to sign customs forms with the Return Address Name listed in the Ship From Location selected for the order. For more information on editing your Ship From Locations, check out this article.