When you create a listing it will ask you to choose a shipping service.
Choose USPS flat rate options, then enter the dimensions & weight
It should show you the cost once it has accepted the details as a preview of what your buyers will see. Then when you save the listing, the price will be included when the buyer completes the purchase.
I sense you might be encountering a problem with this so give some details on where you get stuck and we can try and help you.
