How do I add Customs Declaration Forms to my Shipping Labels?

[sespinueva]

Hello there. Could someone please help me? I am currently running a shop here on Shopify and the first order I got was to the United States. My island, Guam, is in the United States but for any package or item that needs to be sent out from our island needs a customs form. When I was purchasing the shipping label, it didn't have a customs declaration form added to it. I tried to add it but the option did not show up to me. Could Shopify fix that? I ship items less than 16 ounces and I do not want to wait in the post office to ship items and to ask for a customs declaration form. Thank you! 

P.S. Could someone reply with a shipping software that accepts PayPal? Please do not say USPS.com.