Dealing with the elderly presents its own set of challenges. She may just be cheap, but also may have talked to someone else about servicing her lawn, and actually is confused. If you can live with the lower price, I'd just let it slide.
We mow for a lady who is 93. It is a small yard, so I cut her a break on the price, and just charge her $100 a month, even if I do 5 cuts (normally $25 per cut). She likes to pay cash, so you have to wait until after she gets her pension and goes to the bank the first part of the month. Half the time she is asleep, and won't answer the door. Last month I only mowed one time due to the drought, but she still owed us for July. I finally left a note on the door, and she called and left a message on the answering machine that she was sorry but she goofed up and didn't have enough money in the bank to pay us until September. I mowed it again last Monday, but couldn't get her to answer the door, so left a note and told her to just mail it. Still no money.
She once told my wife she just didn't understand why the guy who mowed for her before quit doing so?