The Post Office requires that customs forms be filled out online at https://cns.usps.com/cfo/ShippingInformation
Some hints on filling out the customs form:
a. Make sure to choose the correct class and packaging.
b. The weight on the second page is the weight of the contents only—no packaging. It’s okay to estimate the weight.
c. In the AES Exemption cell use NOEE1 30.37(h). Remember that APO/FPO addresses require a customs form even though they are considered domestic for postage costs.