I beleive you CAN ship by USPS although may have to pay hazmat charges. I could be wrong though because I only ship ammo through UPS, there are no hazmat charges (unless you are buying powder) and because ammo is heavy, the shipping costs much less than USPS. I do not think Fed Ex will accept ammo at all.
For what it's worth, usually the shipping costs will kill the savings you get from buying ammo online since it's so heavy. This is unless you buy in bulk or are buying something rare. I did an informal poll on my facebook page asking how much ammo someone would buy online, and it seems 1,000 is about what most people would be comfortable buying bulk.