The large envelope is a non tracked service so eBay does not sell that kind of label. If you need to buy your shipping off of eBay then don't select that as an option. It is for people who are going to buy that service outside of eBay (so that the buyer is charged correctly and is aware of how it will be shipped).
It would sort of be like eBay having an option for a stamped letter and if you selected that option you would be required to put a stamp on the envelope yourself. You wouldn't be upset that you couldn't print a stamp off eBay.
