Hi Deth,
I don't believe there is anything specific you need to enable within your eBay or Auctiva accounts to allow you to send combined invoices. Since these items cannot be combined on a single invoice, the only way to invoice your buyer for them all at once would be to arrange payment separate of checkout, perhaps via a Paypal money request.
Ebay does not allow items that use different checkout systems to be combined so, if a buyer purchases an Auctiva checkout item in conjunction with an eBay Checkout item, unfortunately, they will not be able to be combined onto a single invoice. However, if the items you wish to combine use the same checkout system, whether it is eBay’s or Auctiva’s, they will be able to be combined successfully.
Any listings you have posted directly through eBay, as well as any listings posted through your Auctiva account when Auctiva Checkout was disabled will use eBay Checkout, while any listings that were posted through Auctiva while Auctiva Checkout was enabled will use Auctiva Checkout. Items relisted through Auctiva while the Auctiva Checkout setting is enabled will also use Auctiva Checkout. On the other hand, items relisted through eBay will use eBay Checkout.
If you have any other questions regarding your Auctiva account, please feel free to contact our customer support team using the web form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx
-Mike