Tipping Caterer using their rentals

Gratuity is the food and service not on additional item rentals, 15% is a good amount because most places charge up to 20%. So don’t include the napkins.

If he is giving you one invoice with the gratuity and 1 invoice for the napkins, no you do not add both invoices to increase the gratuity. Pay him for invoice A and pay him separately for invoice B.

If the caterer said they are charging 15% gratuity, the amount will be listed on your invoice. You don't need to tip above what is listed for gratuity. You can if they go above and beyond but its not really necessary.

From my experience the gratuity that is added in the contract is for everything. So what ever is your total bill, add tax, then add tip.

But it could be different, you should definitely reach out and clarify with your caterer.