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 the

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.