I did the stamps.com account. It's 15 a month to maintain the account plus whatever postage you need. I got a free postal scale that plugs right into the computer and weighs right into the software that you download. It's helped be a ton so far and i'm really in love with the fact that the only reason I need to go to the post office is to drop off my packages. Beyond that they credited my account $5 automatically, and then I got an additional $20 in free postage for mailing back two postcards with stamps that I printed from the software. If you don't feel like doing something like that buying a cheap postal scale from ebay or craigslist would work and USPS.com has a postage calculator that you can plug in where you're sending to and how much the item weighs and then they give you a breakdown of all of the options you can use to send. Still would have to stand in line at the PO but at least you would know what to charge for shipping.

bluecalla, thanks for that! I joined Venture for the shipping reductions and Canadian upgrades, but it is so confusing to figure out shipping rates...plus they measure everything as well as weigh it-got any suggestions? I'd like to list shipping to other places besides N.
America, but I'm a bit nervous about guesstimating...will try the scale for local stuff, but have no clue how to figure out foreign shipping yet, and i know that people won't bother convo-ing you if you don't have it listed..*sigh*

FYI - USPS will pick up parcel post packages as well.  As long as they have postage printed out if they are more than 13 oz.  You just go to the USPS website and schedule pickup.  There is no charge.