I take care of people's cats and dogs in their own homes when they're away.
I just was called by a neighbor who've I've helped out a few times recently on an emergency basis. She and her husband were going to the local mountain for the holiday weekend and one of their cats just didn't want to come in the house due to their nephew's Great Dane being in the house.
Long and short of it - the cats are terrified of her and her taking over their house.
My fee is $10.25/cat/day. If there are two or more cats in the house I charge an additional $7.25/cat/day because I'm not traveling anywhere extra.
She hired me for the holiday weekend (Friday-Monday). I asked her if Dusty was going to be alright if he was on his own if she took Butterscotch with her to the local mountain (those two cats are always together). She went, "hmmm..." and then said she thought it'd be best for her to leave Butterscotch as well just incase.
So it went from originally one cat to two.
She came to my front door this evening on her way to her yoga class that she teaches every Tuesday evening and dropped an envalope off for me. It contained $32.
I don't know if I should ask her for more money or just keep it the way it is and just remind her for next time of the proper amount? She's been so kind to me, that I feel badly just asking her for more money. If you think I should, how should I go about it to make it as gentle as possible?

I have a similar side-business of pet sitting for some people when they head out on long trips. They have 2 large dogs, 5 cats, countless koi, 2 rabbits and a guinea pig. I don't run a set amount, really, since they do pay what's fair and I offer to take a scratch less than any kennel would offer them. But in your situation, I would say to go with just eating it this time, taking the amount she offered and then remind her of the rate next time. She's been kind to you, as you said, and any issues with money can be tricky. But perhaps if you remind her next time of your fees, then she'll realize she didn't pay you full last time and throw in extra?
And perhaps when you asked about Dusty being okay without Butterscotch, she somehow took that as you being fine with watching both cats without the set fee for the additional one. Or if you WERE to bring up the money situation, she could become offended to some point since you brought up the possibility of leaving the other cat in the first place. Could set you in the light of someone who wanted the extra money. Not sure, only because I don't know these people, but I know how SOME people can get about that stuff. :(
[tot=Demorg]
You are providing a service, and I will assume that she knows your rates if she has used you before. There's no reason she shouldn't pay you the correct amount, otherwise next time she might think she's allowed to pay less because you won't say anything. You don't pay less for a cheeseburger either just because you want it with no salad. This first time, just be nice towards her, tell her that you assume it was a mistake and remind her of your rates. If she does it again, it'll have to be unpleasant. Good luck :)