We have two options when it comes to Buy It Now items. You can either force people to pay for the item when they click the "Buy" button (recommended), or you can opt to have the items added to a final invoice.
Adding items to an invoice is mostly used when running a live mobile auction. It allows bidders to make one final payment at the end of the night instead of multiple payments.
1) Click "Auctions" on the far left.
2) Click on "Manage Auctions".
3) Click on the Name of your auction.
4) Click "EDIT".
5) Scroll down to find "Buy It Now Invoiced" and click "Yes".