I would either accept the payment and then make a donation to the American Heart Association or ask them to make a donation in your name. If not the AHA, another charity that is important to you or to them.
I can understand them not wanting to feel like they are taking advantage of you by paying you. Please try to appreciate it. How many times are there complaints on this board about people who want us to make quilts for them for free? Way too often, so this is a nice problem to have. :thumbup: