I appreciate everyone's input. My first impulse was to send her a check for $300... but I didn't know what the quilting was actually worth. I didn't want to send her more than she would have billed me.
I mailed her a check for $300 this morning with a note that said "The quilting did turn out wonderful. Thank you for doing this for me." I didn't mention her offer of "No Charge." Have I paid her more than she would have billed? Maybe... but I won't go broke either way. I'll let you know if she deposits my check.