In a German invoice, if the quantity is not countable, the term "pauschal" is used. E.g.in the example for good delivery.
Flat-rate is the wrong term, as there is no "rate" for a single action. In Germany, flat-rate is linked with Internet access, mobile services as subscription service.
I found "all-in" ( = "all-inclusive" ) and "lump sum".
What would you suggest, from your experience with bills / invoices of UK/US/AU,.. enterprises?
Comment on the thread starter example, the same: It is a "all-inclusive, one-time fee", not a "flatrate", as there is no sequence of several uses.