All the ppv movie services I have used allow you to either add credit to your account or just pay individually from a credit/debit card.
I have used Blinkbox and have had problems with the audio being slightly out of sync and pausing and restarting solves problem for while. I am currently using my £50 free credit with Acetrax and find it ok most of the time, but Saturday evenings give problems. After a quick Google search this seems a common problem. The most reliable service I have used so far is Sony Movies Unlimited (Qriocity). I have Knowhow, but haven't tried it yet due to the films costing more. Eg. New release SD is 3.99 for some movies whilst other providers charge 3.49. I don't like using Sky Box Office as I like to PPV and not wait for my monthly Sky bill.
I have Netflix, Crackle etc etc, but still like the option of a new release ppv. There is a market for both.