If you sign up to MyPost Business, you can generate labels through there. It will even allow you import eBay orders, so you don't have to C&P the address. As there is no eBay Medium satchel, you would have to be using an AP one anyway.

Unless you are using your own packaging, in which case weight pricing applies. If that's the case, it will almost certainly be more expensive, so I suggest you buy a single medium satchel from AP for 25c.