What would you think to this Customer request ?

I think it is a cheeky request, but it's the way it goes sometimes in business. I think an important question is - does this person have any influence over the decision in continuing to do business with you? If not, you could speak to the person who does make the decision and explain that whilst you'd love to contribute, you'd rather keep your costs down so that you can continue to offer a low cost service to them. (And if anyone needs looking after at that company, it'd the decision maker). I think the whisky gift you already give is fine enough, and you could say to the team leader that you're happy to continue with your usual policy of a bottle of whisky each.