Here's the shipping info for Priority mail. Whatever you can fit in the box can ship for a flat rate. They have different sized boxes at different prices. There is also a priority option that isn't a flat rate. If you know the weight, you can gauge the price. There's a chart for that in the link below too. I'll probably just use the priority box this time.
http://www.usps.com/shipping/priorit...e=prioritymail