Is it permitted to shave a mustache with a razor?

Hello and blessings,

There is no prohibition as a matter of basic law, except at the edges of the mustache, where some prohibit it.

However, see what the Rambam wrote in Hilchot Avodah Zarah, chapter 12, halacha 8: The upper lip may be shaved with a razor, and that refers to the hair on the upper lip, as well as the hair hanging down from the lower lip. Although it is permitted, Israel did not customarily remove it entirely, but would shave part of it so that it would not interfere with eating and drinking.

Success,

Binyamin Shmueli