I also think it is a good idea, and I am a carpet cleaner... And let me tell you why..
As carpet cleaners we often (not all of us and not always) give a good customer or a new customer a bottle of spotter, it is own lable spotter ( I make mine on water decals and print them off and put them on a 250ml bottle) this serves 2 purposes.....
1. it stops them using vanish 1001 etc on stains on their carpets.
2. it is a permanent business card in their house under the sink.
This increases the chance of customer loyalty, and as also mentioned by someone else... Customers like things for free.
So in this instance as a free gift for a first clean it is excellent, and after the 3rd clean maybe you could refill the bottle for them..... After all how much does this water cost?? and how valuable is a loyal good paying customer??