If you select to 'Send an invoice' to your Buyer, this means they cannot Check Out until you have sent them an invoice.
Invoices are generated automatically by WARSTUFF on a listing ending successfully. You are able to modify the final payment amount before you send them to a Buyer.
To send an invoice, go to:
My WARSTUFF > Listings > Successful
Click on the item that has ended successfully, and click 'View Invoice'
You have the opportuntity to make final adjustments to the costs. Click 'Send' to send the invoice to the Buyer.