Not sure if this helps, but if I were in your shoes, this would be my mindset…
Water really isn’t that expensive. I’m not sure what city you live in, but as an example in Phoenix, 12,000 gallons costs $34.29. In Boston it’s $65.47 per 12,000 gallons. A toilet flush is about 1.6 gallons, but let’s assume 2 with hand washing, etc. So even if you’re water bill is tied to your sewer bill, worst case you’re talking 2.5 cents or so per flush.
I don’t know if you advertise, but if you do, it’s to entice people to come into your shop. There are few advertisements or marketing programs that will get people in the door (that otherwise wouldn’t have come in) for 2.5 cents per customer.
I’d look at it as a marketing expense. You’re differentiating yourself because you are one of the few “friendly” places that lets people use the restroom. People want to shop at a place that has a message and a personality. The open bathroom policy becomes part of who you are (you’re not like the other restrictive shops), and I think that is a positive thing. If only one out of 10 people that came in from the outside to use the bathroom bought something, you’d still just be spending 21 cents for a paying customer, that you may not have otherwise had. I’d imagine your bathroom policy would be good for the bottom line.
I’d go one step further - to increase the number of people that entered your Deli, and to let people know what kind of deli you are, you could have a sign outside your establishment that said, “Our bathroom is NOT just for customers - come on in”.
Once someone has gone into the deli to use the bathroom, maybe you can help convert them into a paying customer with signs in the bathroom or in the Deli. (“Long Day? Rejuvenate yourself with an ice-cold lemonade and chicken sandwich”).
Wouldn’t that be neat to try for one month and see what happened?
I recognize that you need to keep the bathroom clean, and there are other costs involved, but as someone that sometimes goes to touristy areas, I find the atmosphere in some of the artsy mom & pop shops a little stuffy, and off-putting. A “bathroom for customers only” sign lessens the appeal of those shops because they seem less friendly and only interested in me if I spend money with them. Guess how long I like to be in a store I subconsciously think is unfriendly? Not long.
Your open bathroom policy in contrast says, “I want you to be comfortable (whether or not you buy from me)”. I have a strong suspicion that message will garner you extra traffic and extra sales that would more than offset your costs.