Lastly, there is a way around everything, legally. Look at OJ! If you take your slide off and ship the slide in one box and the frame in another, even if both boxes are shipped within a larger box (SWOG) you can ship it as parts with almost all carriers. SWOG - Ship with other goods, is a way to manifest the contents of the box, e.g. put a packing slip on it folded up saying box #1 slide assembly for Hi Point (good choice fo returns) and box #2 frame assembly. DO NOT SHIP LIVE AMMO, this is HAZMAT and you will get caught. Our facilities have dogs, detectors, guards, etc...
USPS will not accept SWOG, this is limited to the big three DHL, UPS and FedEx. However the best method is to separately pack and ship on separate tracking #'s. I've done it, I've seen it work. Your FFL is definitely wrong, use the ATF #'s and get a contact name to shove your cell phone up his *** so he can hear it from a Fed.