You can purchase labels whether you are fulfilling a specific PayPal transaction or just sending a personal package.

Buying First-Class postage online using PayPal is a convenient way to access discounted commercial rates that are often unavailable at the post office counter. While the official USPS Click-N-Ship service typically restricts online label printing to Priority and Express Mail, PayPal allows you to print First-Class and Media Mail labels directly from your home or office. How to Buy First-Class Postage via PayPal