I don't use Auctiva checkout for some of these reasons. That and you cannot combine purchases using Auctiva checkout. If want to disable it, go to "My Account" then click on "Settings" and then to "Checkout Settings". Un-checkmark the preference to use Auctiva's checkout.
For this sale, you may just want to send them an invoice through ebay so that they can pay via Paypal.
Hope that helps....just holler if you have any more questions...or of that did not help.

Marcia