EBAY SHIPPING BOXES HOW DO I KNOW WHICH TO PUT ON MY LABEL I PRINT?

[Jessica]

im brand new to selling on ebay, I just sold an item and I need to ship it. I got a variety of those free usps priority mail boxes on the us postal service site. I am going to print my label at home. but when I go to select what the label is going to be, I have no idea what to put for the box/package im going to ship it in. im shiping it through usps, and there are more than several options to pick, I have boxes, but idk which is what, one option is a small flat rate box, what is a flat rate box and how do I know if I have one? I know the mesurements of al boxes but I still don't know what to select as my option on the type of box Im shipping it in, which in turn gives me my shipping label price/cost. so I don't want to put the wrong thing and it cost me money, also what happens if put the wrong thing? please someone help me Im sooo clueless about this, I figured it would be easy and so far it has been but this is the one thing that im stuck being clueless about. all info is apreacieted thanks

[Christina]

allah will provide