You tell them straight up in the sales office. Then you tell them again that it will reflect in the contract, and then you point it out again in the contract, and then you point it out again when you close out their bill.
When I say straight up I mean straight up. Just be honest and don't feel like you have to have an excuse. It's standard.
I write on the contract "Does not include gratuity"....when clients ask how much is standard I say 15-20%. The state asked if I included gratuity in my bid and I said "no" gratuity is that.
My staff is paid $15-25 per hour with 4 hour minimum, tips are on top of that.
As a "Personal Chef" I always add 20% gratuity (4hrs minimum/$25/hr) for my servers or bartenders, never for myself, I leave that up to the client.
My bill reflects the total $ amount for my services (shopping time and in home service), the ingredients costs and servers costs as I do not charge according to a food cost analysis. The total of the three amounts of course is the grand total of the bill.
My partner and I have a slightly different opinion on how this should be handled but we settled on only charging a gratuity when we have to hire people to pass. We charge $25/hr per passer with a 4 hour minimum + 18% gratuity. If we do not have passers we let the customer know tipping is optional. Personally, I think charging a gratuity is an oxymoron and another way to gouge the customer. If you are worthy of a tip (and the customer is not new to the ins-and-outs of hiring a caterer), you'll get one. And yes, we have done jobs without receiving a tip, but 80-90% of the customers end up tipping us 10-20% anyway.
We do have a spot on our invoice that shows "gratuity" so when the customer reads the invoice they see that and hopefully a light bulb goes off and they tip us.
Not all people will be nice and tip anyway. In some larger operations like hotels, gratuity is part of the bonus. First all of the tips from catering are pooled. Then everyone takes their cut. 2% for the sales director, 1.5% for chef, 1% for catering manager, etc. The rest is split out among the servers.