Greetings,,,
You use standard shipping labels that you can get from any office supply store, or that thing they call the internet. If you look at anything sent to you from a coop, you'll probably see that they are usually the half sheet size.
This is why the coops require you to mark "Goods" in Paypal. That sends along the member's address from Paypal, so it can be printed out on the labels, without requiring it to be done by hand. The labels make things much easier for the coop manager and allow less margin of error, since it doesn't have to be done by hand, but does cost a little bit for each label.
Keith