I tried purchasing a £44 VST with a £50 voucher. The invoice states that the item is 'free', yet it seems I can't complete the transaction without selecting Paypal, Klarna or credit card details, is this correct?
Yes it is correct. But it is just typical, whatever you choose, you will see 0 amount to pay.
It's not that much of a stretch to think their e-store is not setup to handle negative balance transactions. What happens when you enter your info and hit submit?