IMO, this doesn't seem that bad, given that professions are a minor income earner compared to other things; If a Courier in Vallaki is using magic on the down-low to be outstanding in his field and earn some extra gold, that seems entirely legit -- So long as it's roleplayed as such. You're being paid more for casting those spells because different jobs will pay you bonus wages based on your attributes/skills relevant to the job. A pleasantly mundane usage of magic to subtly enhance ones' personal life.