A single Breyer Horse won't weigh enough to quantify using a flat rate priority mail box. If you use a regular box, one horse won't cost more than eight or nine dollars, even to California.
A flat rate box would only be worth four lbs or more really. Even with packaging, the horse won't weigh that much. I buy Breyers a lot, have over 500 in my collection, and one horse out of the box with packaging may weigh two lbs max and I think even that would be pushing it. My suggestion since you still have three plus days to go on the auction is to see about packaging the horse up even if you don't seal it and put it in a box, weigh it and see what the weight would be. It will help when others are asking you for a shipping total. And may help increase how many credits you get with the shipping lower than the flatrate priority mail.
trust the person TheJRLoveless its true dont do the post office boxes just get a regular card board box to fix horse and it will be cheaper to send for the winner