I've ordered legs from PBR several times. They will cost you around US$150 for the shipment. The box fits 16 legs (4 sets) and is sent by UPS. USPS will be around the same price, but will take a bit longer to arrive.
I would email Steve at PBR to confirm, and really, it is only worth your effort if you need multiple sets. You can simply buy the WPC 28 1/2 ribbed legs locally and they will look fine.
Finally, there would be a good chance someone might want to swap your 31 inch legs for a set of 28 1/2 inch legs. I'd try that route first.