Why would someone send a text message telling her to remember to pay in the future? My business is just a small business.

First of all, you have to find out whether she didn't give it on purpose or whether it is included in the free package. Send her a message saying it's the best policy. Next time you come to do your hair, politely tell her how much it was last time and how much it was this time. And said that the next time to do hair discount, how much discount, decide for yourself, don't let her feel insincere, what you said is not good, very useful, I hope to adopt it.