How do I politely ask a client if they would mind requesting more services as per what they asked me to do?

I realize that the wording of my question is a little confusing. I am a first time walker and a new client just sent a request for three walks. I said great! Let's set up a meet and greet. The owner replied and then informed me that they would also like me to let their dog out in the mornings and at night (Not included in their request). I am happy to do so however, I don't have a car and Ubering three times a day/taking public transit would be expensive. How do I politely tell this owner that what they are asking for is a bit more expensive than three walks? This is my first request and I don't want to mess it up. The other issue that I am not sure how to raise with this request is that the dog is 7 months old and will be left alone for the majority of the day. I would offer to board however, my dog is just getting over kennel cough and I don't want to get another dog sick. Any advice would be greatly appreciated!!