I haven't encountered this specific problem, so I can only guess (hopefully there will be someone else to help out soon), but are your items the only ones she has purchased and / or is she using the cart with other items in it (possibly not yet purchased)? 

If she is trying to pay directly via the invoice, I'm not sure what could be causing the issue or how to fix, but if she's attempting to checkout with other items in the cart, there might be an issue with another item she has in there. 

If it can't be resolved via eBay, the only alternative I can suggest (if PayPal was offered and being used) is generating an invoice directly from PayPal and then marking as paid once payment is received. Just be sure to include the eBay item details in the invoice.