Upvote:4
You still need to declare what you are shipping, whether it was bought in the USA or EU. Customs at the destination will make the final determination if duty is owed and they will instruct the local postal service to collect said duty when the parcel is delivered to you at home.
If your camping gear is a EU brand and well used, then likely customs will pass it through without duty. If it is a North American brand in new condition, they may not.
Customs inspectors are pretty well versed in determining what is new or used, what is from outside the EU, what is not. But from USPS's (or FedEx or UPS or DHL) point of view, they don't want to make this determination and will simply gather all details to forward with the package so that the customs department can.