I didn't think you could get first class over 13 ounces even if you requested it, that you had to use priority over 13 ounces. (?)............. Incidentally, the flat rate Priority mail (something like $4, and you can put in all that fits) is a rip-off. You can't put much in that flat envelope they give you, and (my post office, at least) you are not allowed to tape the envelope closed with your own tape. You can only use the thin strip of tape that comes with the Flat Rate flat envelope they provide.
Robert