You are right to want to pay her something, but the amount of the check should be tempered by the anxiety you felt in having to wait so long and whether you know how unavoidable the delay was. It seems communication was lacking.
I'm going to go against the flow and object to a gift to charity. This is someone you don't know well, and there's every reason to assume her choice of a charity wouldn't be the same as yours. I don't think it's appropriate as compensation for work. It's okay for some situations among people you know well, such as when someone dies and the family would rather name a charity in lieu of flowers.
Another idea: Your piecing is so extremely good, that I'm wondering if she might not be thrilled to accept as a thank you a small top that you might make for her. Both of you would appreciate the value, but it would have the advantage of not being defined merely in dollars and cents for either of you.