If you ship firearms USPS, then you need to fill out form 1508. If your post office is not familiar or has issues, then it can be a hassle. New clerks can be a hassle, until they are educated.
I have not had a problem shipping USPS. It is much cheaper. Also, create an account on the USPS web site and order (free) regional boxes (not available any other way), which can be much cheaper than the flat rate boxes. You still need to compare the rates to see what the lowest rate is.
FFL Transfer/Special Order Dealer since 1993.
Net-FFL list maintainer.
Never try to teach a pig to sing. You waste your time and you annoy the pig. - Robert A. Heinlein