Yes, I need people to come in daily. My cats are quite old and they drink a lot. Their water needs changing/topping up every day. They're indoor/outdoor so there's no litter tray and they eat dried food, but they still need someone in daily. If the cats are on wet food then they definitely need feeding daily.
I'm not saying the OP is being cheap and I agree that the bottle of wine is an equivalent cost of 10 days worth of petsitting if not a bit more. The problem is that the friends *don't know that*, if we assume that they have no idea of the value of the wine that they've drunk. So, if you suddenly back out of taking them to dinner and a movie and give them a Starbuck's card instead, there's every possibility they're going to think "Wait, we went to their house every day for 10 days and they got us a cup of coffee? That's the last time I do *them* such a big favour!"
That's why I think it needs to be explained, awkward as it may be.