The company I use to do all my fulfillment is Speaker Fulfillment Services.
Bret Ridgway, the founder and CEO of Speaker Fulfillment Services, has a great article on his blog about integrating Paypal and 1ShoppingCart.
It's a simplified set of step-by-step instructions.
If you're interested in taking payments via Paypal, and you use 1ShoppingCart (or any of its many private label versions like Marketer's Choice, Website Automator, and 1AutomationWiz), you will find this article very helpful.