We're having a cash bar *gasp!*. Cocktail hour may or may not be hosted- we haven't decided. The bar accepts credit/debit in addition to cash. I wanted to let guests know in advance so they're not surprised when they arrive. I plan on including something about it on the website, but I'm not sure how much detail to give. I have a super basic price list (ie Beer- $5, Wine -$6, etc) that I could include, but I wasn't sure if that'd be too much info or come off weird to include prices? It's almost all family (50-60 people) but I don't trust word of mouth with my own side of the family, at least. We're also considering a sign at the bar itself in case anyone *still* misses the memo, but I'm not sure what to write seeing as a sign just saying "Cash" bar could be misleading since they accept cards, too.
To those of you who had a cash bar or have been to a wedding with one, how did you let guests know, or how did the couple let you know? What did/didn't you like about how the information was relayed to you?
You can go on all you want telling us why we shouldn't have one, or have a modified or consumption until you're blue in the face- it's what makes sense for us and what we've chosen. Done and done 
