We're in the process of translating a website from Dutch to other languages, including English. The texts were translated by a professional translator, but I'm not sure one of the words has been translated correctly, maybe caused by lack of sufficient context.
The word in Dutch ("eenmalig") is used to explain there is no monthly fee for a product, there is only a one time payment, after which the coverage lasts for years. This in contrast with other products having a monthly fee.
There are for example two options, displayed as follows:
'4 years: Premium £ 9.99 one-off'
'5 years: Premium £ 11.99 one-off'
I came up with 'one time only' or 'once' as alternatives, but I'm not 100% sure.