If you ship using weight/dimension based Priority service on the label then it doesn't matter if you use a (non-flat rate) Priority box or a plain brown box.
If you ship with a Priority flat rate service on the label then you must use the equivalent Priority flat rate box or envelope.
If you ship in a Priority flat rate box or envelope then you must select the equivalent flat rate service when buying the label.
If you plan to use a plain brown box then you may find Ground Advantage rates are more competitive than Priority rates. You cannot downgrade service though. If you plan to ship Ground Advantage that has to be offered in your listing. If the only service offered in your listing is Priority then you have to ship using Priority service.
There is no cost to buy the label through eBay other than the actual cost of the label. If you make a mistake on the label (like entering the wrong weight, dimensions or service), you will be charged a cost adjustment by the carrier through eBay after the carrier physically processes your package.
Make sure you enter correct weight and dimensions when you set up the listing because that information will determine how much the buyer pays for shipping when you use calculated shipping.
Verify the weight and dimensions are correct before you print the label.
