Sorry, Arcom, but you've been mislead on this matter.
The frame (receiver) IS the weapon. Taking the slide off doesn't make it less a weapon from the US Postal Service perspective.
Indeed, if it were that simple, people would be sending guns in two packages all the time, by insured Priority US MAIL.
DON'T SEND IT BY MAIL. That's a potential felony!!
The postal regs say an individual (non FFL) can't send a handgun by mail. Some military personnel can. But even 03 FFL holders -- Curio & Relic licensees -- can't ship C&R handguns by mail!
There are no exceptions.
You CAN send it by by UPS or FedEx to a licensed recipient without a FFL involved at the sending end, however, and that's probably the route to go. The manufacturer is a licensed recipient (and has its own FFL).
The manufacturer can return it the same way to you, without FFL involvement on the receiving end.
If you're friendly with a local FFL, however, that FFL can send it through the mail, and its about HALF the cost of sending it by UPS/Fedex. Got a friendly FFL? (I've shipped several guns that way, at about $12-$16 a gun.)