Many people saying here "Pay once, fishy, not trustable". I don't know how much this critic relates to the topic for each commenter (invoicing, being a constantly changing field) but how the general public loathes subscriptions and binding, I actually would double-triple consider any subscription-based software (I'm speaking generally here, not invocing) and would always choose the one-time payment over it.
OP: there could be a better middle ground - you could still offer a one-time payment, and optional subscriptions to newer versions (eg. conformity to newer regulations).
OP: there could be a better middle ground - you could still offer a one-time payment, and optional subscriptions to newer versions (eg. conformity to newer regulations).