Albert Barnes on Luke 3:13

Exact (k). Demand, or take, no more.

Than that which is appointed. That is, by the government. John does not condemn the office, or say that the employment should be forsaken. Though it was hated by the people--though often abused and therefore unpopular --yet the office itself was not dishonourable. If there is a government, it must be supported; and of course there must be men whose duty it is to collect taxes, as the means of the proper support of the government; and as such a support of the government is necessary, so the people should pay cheerfully the just apportionment of their rulers, and regard favourably those who are authorized to collect it. See Rom 13:1-6.

(k) Lk 19:8, 1Cor 6:10