I'm a longtime collector of 78 rpm records, and have bought over 400 through the mail. They're pretty fragile, and sometimes I get broken records because of inadequate packaging. If you're interested I know a method of sending them that never fails, and I'd be happy to share what I've learned (sadder but wiser!).
Place the record between 2-4 squares of corrugated cardboard (for stiffening), then wrap it tightly with packing tape. Place the sandwich in an oversized box with lots of padding above and below, so it doesn't move around while in the tender care of the USPS.
This always works, and the materials are cheap! The cardboard squares can be cut from old boxes, and crushed newspaper balls work as shock absorbers if you don't have any packing peanuts.
I've bought around 400 78's through the mail, and I've received several broken records because of inadequate protection in packing. But I've never known the sandwich method to fail. If I win the auction I'll gladly pay any expenses you might run into with the packing.