There has been a recent question where the OP later specified in the comments that he's open to paid software as long as he pays once for a lifetime.
Even the Apple's App Store now has a chart for "Pay Once & Play" apps. Source: Apple has a new section in the App Store to highlight games that don't make you keep paying to play
I feel that this is a common enough case to deserve a tag. What do you guys think?
My suggestions for this tag are single-payment or single-transaction