Yes you can just pick them up at the Post Office. They have the flat rate envelopes for 3.85 and they also have the flat rate boxes for 7.70, the boxes are great, you can fit about 500 brass in the boxes and it's only 7.70.
You could put lead in them if you wanted, the weight limit on the envelopes and the boxes is 70 lbs. It's a great way to ship reloading supplies. I bought 150 270 bullets awhile back and the guy just put em' in a zip-lock bag and shipped them in the flat-rate envelope.
Usually the post office will give you several of these boxes and envelopes as well as a roll of USPS tape for free. Then, all you have to do is print out a shipping label online from the USPS website and they'll charge that flat-rate fee to your credit card and have your post man pick it up or you can just buy the labels at the PO. It's a pretty good service.
You can just email me your shipping address and I'll get this payment in the mail. firstname.lastname@example.org