Very few water pressure issues around here, so using the HO water is not a problem in that respect. Some HO's don't want me to use it, but when I tell them it is an added cost ($25 and up) to bring my own, they usually decide to let me use theirs.
I always try to upsell. You won't know what they are willing to have done if you don't ask for the business, or at least suggest other services. I have gone back to do more services for people who I tried to upsell, but weren't ready. I always let them know that if I do multiple services on one trip it will cost less than having me come back multiple times.
If I get the upsell, I will always try to find a little something to throw in for free. I washed an elderly ladies car in July as a freebie and she was so thrilled, she told all of her neighbors. I got 4 more jobs within 2 weeks from that little thing. The car took me less than 10 minutes while my daughter was still rinsing the plants.
Try something like that. It can pay off.