the mag is not technically the gun and doesn't have to be transferred, remember. Wherever the serial number is technically what the gun is. Just take the mags home with you and send just the gun for transfer.
I've transferred just the frame of a USP before during a face to face sale, keeping the rest of the gun, with the box, at home. You can even have anything that's not the receiver (or in this SPECIFIC case, the mag) sent to your front door since the only part that you need an ffl for is the gun which is technically only the receiver, or wherever the serial number is printed.