I think when they pay you through PayPal and specify that the payment is for goods received (not a personal transfer of money), then there is a click link to create the postage label for shipping (which is very easy to do).
You can also create shipping labels through PayPal for packages you want to send that have nothing to do with PayPal. In that case, after logging into your account, click on "Merchant Services" at the top of the screen and then on "Multi-Order Shipping" to get to the postage area. I often use this to print postage for a single package. When the package is not associated with PayPal, the only difference is that you have to manually enter the receiver's address information.