If you don't have access to a top-of-the-line scale and postage meter, sending snail-mail can be a real drag. (True confessions: I recently had an important package returned because I'd guessed the weight and underpaid the postage. The resulting fallout made me wish that SNL's Jiffy Express were a real company.)
Using an all-in-one service like the one offered by stamps.com is tempting, because it includes a free scale and allows you to print postage directly off the internet. But the privilege costs an extra $16 a month. A cheaper-in-the-long run alternative is simply buying a digital scale like the DigiWeigh DW-36XP ($30 from Amazon) to weigh your packages (up to 36 pounds). Then just apply your own stamps based on the ever-changing prices of the USPS. If you really get into it, you can have Zazzle send you custom stamps bearing your company's logo for a small markup. For packages, the postal service also allows you to print priority and express mail shipping labels. It certainly beats waiting in line at the post office.