OP; The 54cm and 44cm measures are the seat tube sizes not the wheelbase. And did you mean UPS instead of USPS?

On the side for shipping, highly recommend using Greyhound buslines (if available) as they are cheapest and less likely to damage a well packed tandem.

If you use FEDEX, start by calling them on the phone and telling them you have a "Tandem Bicycle" to ship. That is encoded somewhere in their help desk screen and if you give them the magic code, you will get a much better price that just shipping a box of said size. I have never found a way to use their webpage and to get the "better price" for a tandem.

Highly recommend NEVER use UPS to ship a tandem frame or bike. If you need pictures of destroyed ones, PM me and I can fill your inbox. For most other things, they are adequate shipper but tandems seem to be outside their expertise.

BTW; if you search the bike forum for "shipping a tandem" there are many threads to read through.

/k