"Sure Lucy." There, I did it. What was the hard part? The part where you say yes or the part where you comp your most valuable customer six bucks in flavored water?
Good customer service starts with accepting that you are no longer an employee and that any rule causing bad customer service is your personal fault, because ultimately, you wrote the rule.
See also a customer walking in and offering you six bucks cash for the Groupon deal. That is strictly better than selling her a Groupon for six bucks and paying $3 to GroupOn. Yes. I think we can make an exception for you, ma'am. Just this once... and for anyone else asking the same.
Edit: Worth the read for description of a sales call between an unsophisticated consumer and a business with 90% margins, though. (cough Who do they think they are, coffee sellers?)