Hi blueskies,
Hmmm...Since Auctiva checkout is integrated with eBay checkout, I believe when you send an invoice through eBay, when the customer rreceives it they are directed to the Auctiva checkout. I don't see why they shouldn't be able to pay in that manner.
If you find out what error message they are receiving, please file a support case using the appropriate link on our help page to let us know.
http://www.auctiva.com/free_auction_software_help/help.aspx
-Mike