Seriously, if Grandma doesn't bring the money, then I would pay a visit to their home this evening requesting payment - with your husband. I would keep it friendly and listen to her excuse for not being able to pay and then let her know in the same friendly manner that if payment is not delivered this evening by xxxx time then you will have to make the business move of terminating her contract and sending her account to collections and then walk away.

I know some of you will say not a good idea, but if you stand up for yourself in a business professional way then perhaps they will see you mean business and are taking control of it.