Hi Lara,
I'd keep it short and simple. There will always be customers who attempt to push boundaries and there is nothing wrong with you being assertive regarding your rates. I'd certainly explain that I was grateful for her business and greatly enjoyed her pup. Let her know "Unfortunately I am unable to lower my rates any further."
If she presses for an explanation, explain that your rates are set in a way that allows you to successfully operate your business to best benefit your clients. You could offer examples of paying for cleaning supplies and treats if this makes you more comfortable.
Ideally as our sitting services grow, we are able to increase our prices. If you were to offer lower rates with every repeat client you'd have no openings for new dogs AND be making very little an hour (like you mentioned.) At the end of the day, you have already given her a generous discount and have a history of being reliable and trustworthy. I can certainly understand it being uncomfortable to have these conversations with clients, but they are a necessary evil. If she continues to feel that this is unfair/unjust etc. perhaps it is no longer a good fit.
Hope this helped in some way.
Take care,
Olivia
I recently spoke Rover about discounting and they are ok with it so long as you charge a min of $10. New client wanted big discount for 21 days, booked and cancelled next day! At Xmas time! Pissed!