"If you want it, buy it."
Yeah, the linguistic terminology is not ideal. Imperative mood is never, per se, an ultimatum (an "or else" command). Context may make it so, but just using the imperative prefix does not make it so. It's just a way to mark the verb as an instruction rather than a description. I would much prefer the linguistic term be "Instructional mood", but I'm fighting against a large number of people following long standing tradition.
On an additional note, Klingon does not tend to use imperatives for ultimatums. For instance, a demand for surrender would be worded as bIjeghbe'chugh vaj bIHegh and a shopkeeper might say bIje'be'chugh vaj bIHegh.