Have you ran the numbers on it to see exactly how much extra money it's costing you?
If it's not something huge and too time consuming then I say big deal, do it. I get asked things like spraying off patio furniture, PWing some concrete steps, or sparying out a bird bath sort of stuff, and for what it actually costs me I say NO PROBLEM! The cost of it is well worth the investment of gaining a customer for life, that customer doing word of mouth advertising for me, and referring people to me all because they got more than they expected. It's called customer loyalty.
Up charge some little old lady to spray off her patio or clean up some plant pots and I guarantee she won't be calling you back the next time around. What's that worth?