Flat-rate is usually a pretty solid bet for domestic shipping. It gets there quicker than UPS in most cases, and they hook you up with the box. I just always forget to buy more shipping tape. They have envelopes too for smaller things, and those ship even more cheaply.
Assuming that we are talking about clothing here, unless you are shipping from coast to coast the Priority Mail mailing boxes (weight and distance) will be less expensive than Flat Rate about 90% of the time IME.
Small Flat Rate box is never the answer for me. If it fits in that box it will go First Class.