The lovely lady I babysit for constantly overpays me, how should I react?

I'm sure this sounds silly to type out, but I recently started watching over two kids for a lovely family. My rate is $18/h because of commute and how young the kids are. However, I noticed that she has paid me $20-25/hr every time and I have thanked her a few times for it (also, because of this I have been able to pay bills that I was long overdue on so not complaining) She told me to not stress it and she wants to take care of me while i take care of her kiddos.. But how do I react to being overpaid each time? Do I keep saying thank you over and over wgain and texting her about it, or do I leave it? I also don't want her to feel like she is expected to give me extra.. idk :p just a small need for advice on it

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. Edit: Hello all! I can't thank you all so much for the support and generosity. I was not expecting this much attention to what felt like a silly little question, but oh boy! I love y'all! A lot of you are saying I should raise my rates, and sometimes I do agree. However, I like being cheap for those who cannot really afford much but desperately need the help, that's what I advertise my page as :). I've decided I'm going to buy her a gift, maybe a really nice wine as it seems like that's what she likes. I will definitely look into why I always feel so weird getting extra money, because a lot of the time I do think "I'm not THAT good, why are they treating me like this?", but I'm starting to see the ways that I do go above and beyond on things that are not asked of me... and you're right! I am doing a pretty damn good job. Thank you all for pointing this out to me, I will stay humble, (but not to a point I talk down to myself). And I will recognize my self worth. I couldn't have seen it without y'all, you've even helped me in other areas of my life with this advice too... thank you again for the overwhelming love. Good morning, night, and life to you all <3.