For one time shipping, printing on paper and cutting it out and taping to the envelope is fine. There are adhesive-back labels you can buy, usually two to a page, but you have to buy a box so I'd just use plain paper for your shipment.

You do need to set up a paypal login with a password but anybody can do so. Start with paypal.com.

You can also print from the USPS site and print the same kind of labels, but I find it very frustrating to use. PayPal is the way to go.