There is an ever ongoing fight between those who mark gratuity on the bill.
If you call it a service charge and standardize it it a logical fashion then I'll pay it, but if ou call it a mandatory gratuity, you have missed the meaning of the word gratuity IMO, gratuity means "freely given" and if you included it in the bill as mandatory, then it's not "freely given". A circular argument ensues after this point!
If you have to constantly charge a mandatory gratuity, in order to cover your costs, then you're not charging enough in the first place.
I don't at this time cater, but I have in the past and I will in the future, as a client I'd prefer to pay an honest bill and then be allowed to add a gratuity of my choice, that is after all the meaning of gratuity. If there is need for a set up or tear down fee, or an additional transport fee because of the distance I made you travel, I'm happy to pay it all as long as it was part of the agreed quote and the work I contracted for.
But I won't use a caterer that can't discriminate between forced and free, that's just the way I am.
Just my 2 cents!
"There is no such thing as a little garlic." A.Baer