Many seller print out a packing slip that gives the item, the seller's name and address and the buyer's name and address to put into the package.
I would strongly advise using the Paypal shipping labels. Not only do you get a discount of from 5% to 18% on many services off PO counter prices, but they simplify your bookkeeping.
This doesn't work if you are able to use Letter Rates for shipping. PP doesn't print those and you will need postage stamps. However, even those are available from some sellers at a discount.