Informal Wedding - Guests Pay

We are planning an informal wedding for 35 guests. We are getting married in the morning, having a champagne reception on a beautiful boat and then having the wedding meal in a very nice restaurant in the afternoon. We have indicated that the wedding will end after the meal (at around 6pm) and that we (the bride and groom) will then spend the evening in the hotel bar/restaurant relaxing with anyone who wishes to stay longer for a few drinks.

We are, of course, paying for all of the above.

Our issue is with regard to the evening.

We have made it clear that the wedding will come to a close at the end of the afternoon. We are also comfortable that people who do decide to stick around for a few drinks will expect a cash bar (given that the bar/restaurant is open to the public and we are simply having a relaxing evening there).

However, we want to make it clear that should people wish to eat in the evening (as indeed we may do), that they will have to pay for themselves.

What is the best way of approaching this?