Hi Luke,
I guess one option is modifying the tickets so they are of the “shipped” type and see if that works.
Based on what you’ve described, though, it sounds as if SagePay simply doesn’t support any virtual products. Is that accurate/do you find the same problem occurs if you create a test product (again, of virtual type) independently of our ticketing addon?
If so I could only suggest you approach the official Shopp support team for assistance – as it seems odd that a payment addon would not support a key product type in this way, unless this an advertised limitation or something of that order.
Let me know what you find out!