I'm with itlCLEO, consider it return karma and do what feels right.

When visiting my wife's family back east, I never get a chance to pay for anything, and never has her uncle allowed me to pay for my spot on his charter fishing boat.

It is always fun, but it makes it a lot more enjoyable for me if I can contribute in some way. You gave these visitors a chance to do something that many Washingtonians have never had a chance to do, and they likely enjoyed it more because they got to choose what they felt was fair compensation.