I've been selling on ebay for over 7 years and PayPal totally revolutionized the payment system. In the days before PayPal, payment sometimes took 2 weeks to arrive. Today, however, most buyers pay within 24 hours, sometimes within an hour of the auction ending. If I send an invoice and haven't heard a reply after 3 days, I send a reminder. Some people either don't check mail regularly or are bidding on so many items that one gets lost occasionally.
As for postage, I agree with Robinrs. If you plan to sell with any frequency, buy a postage scale. Hauling everything to the post office to have it weighed is a pain. With your own scale, you can quote the buyers exact postage, not an estimate. That will prevent a lot of upset customers (if you overestimate and they pay more than postage actually cost).