I agree an HTTP call for every rounding operation would be awful. I would question service boundaries in this case though. This is very domain-specific, but there's likely only a small subset of your system that cares about payments, calculating taxes, rounding, etc. which would ever call a rounding operation; in that case that entire subdomain should be packaged up as a single service IMO. Again, this gets very domain-specific quickly; I'm making the assumption this is a standard-ish SaaS product and not, say, a complex financial system.
Depending on what functionality the money service handles, this could become a problem.
For example, one example of a shared library type function I've seen in the past is rounding (to make sure all of the rounding rules are handled properly based on configs etc.). An HTTP call for every single low level rounding operation would quickly become a bottleneck.