Sure, there's FedEx and UPS too, but the USPS is (for the most part) the most economical.
For one, I highly recommend their Priority Mail Flat Rate for shipping most packages. First, for a small fee, you can track it and even e-mail it to your customer for proof of shipping. Also, if you have some bulkier items or even for simply ease of calculating shipping costs, the USPS Flat Rate shipping is a dream.
As the commercial says, "If it fits, it ships."