"Once Paypal lets you know that you have money in your paypal account and marked completed, go ahead an ship it. the money is yours. "

No, no, no, no no no!!!!!
You are getting some REALLLLLLLY bad advice here!
DO NOT SHIP THE ITEM UNTIL YOU HAVE CASH IN YOUR HAND. Period. Bottom line: if he paypalled you with a credit card, he can do a chargeback on you as long as there are funds in your paypal account. As a matter of fact, paypal CAN AND WILL FREEZE YOUR ENTIRE ACCOUNT if he complains of fraud. He could just say "the paint was not as glossy as I expected" and call his CC company. Guess what? The CC company is NOT going to get stuck, and Paypal is not going to get stuck...YOU ARE.

Go with what your original instinct was...ship when you have the cash in your hand.
And NEVER keep substantial amounts of money in a paypal account.