I have a client wanting day care 2 days a week for the entire school year. How would you charge for this?

I have a client who wants me to look after her dog 2 days a week for several months. She originally booked for boarding for those 2 days which would cost her $30. I charge $20 per day for daycare, so if she keeps booking as boarding she will be saving $10 a week. Is this the best way to do this, or should I try and set up a custom rate?