As others have said, if it will fit into a flat-rate box then USPS is the way to go.
If not - and if it is heavy and going west of the Mississippi, then check out UPS or FedEx.
May, if you are a frequent shipper, you should check out the various flat-rate boxes at
http://www.usps.com . They do have free boxes that are NOT stocked at the local Post Office branches. They have a wonderful board-game box, which will hold a bolt of fabric (still on the cardboard) or three bolts if you take the cardboard out. And they have other sizes and shapes too.