Unless you're a very large company with a large volume of shipping, avoid using Stamps.com!
The amount they've charged me since opening my account far exceeds what they've saved me in convenience.
They'll start you off with a $15.99 per month mid-level account even if that's not the level you require. Once you figure that out and request that your account be downgraded they'll still bill you at the higher rate for at least a month.
My entire experience with this company has been a disaster. You've been warned!