PayPal's main service allows consumers to buy and sell products online without using credit cards or checks.
The postage service will be part of PayPal's shipping feature, launched in June. It will allow sellers to calculate the exact amount of postage required and print a shipping label with the buyer's information. Another feature will allow sellers to verify delivery of their shipments online.
PayPal said its new shipping tools would be available for the holiday season. Financial details of the Stamps.com deal were not released.
Mountain View, Calif.-based PayPal, which wasby auction giant eBay earlier this month, will report second-quarter results Tuesday. Analysts are expecting the company to post a profit of 8 cents per share on sales of $53.5 million, according to First Call.