There is no proper way to announce this on an invitation. I have seen many who add it to the bottom of their invitations, but it is not proper on a traditional invitation.
Oh, sorry. I, personally, have problems with asking guests to pay for any part of a party they are attending. It is definitely my problem. Many people are now including 'cash bars' at their receptions. If this is what you wish to do, I suppose including an enclosure stating that there will be a beer and wine bar and a cash bar provided for cocktails would be your best bet. This way you wouldn't have to use the word 'free' when referring to the beer and wine.
Limited Bar
