Wedding gift etiquette question

One of FH's college house mates is getting married next weekend in Vermont. We were sent the 'invitation' sometime in the middle of the summer and of course RSVPed yes (I put invitation in quotes because to give you a little description of the invite... when I saw it sitting on the kitchen table, I thought it was a 'congrats on your engagment' card. It was a basic Hallmark card that had all the wedding details printed inside with no response card. We had to RSVP to either the bride or groom's cell phone).
The ceremony is on a Friday night and that is private, we're invited to the party/reception the following day, Saturday. My question is, they're registered at BBB, and since it's such a low-key affair (bascially one of our friends called the groom to ask what the atire was and he was told jeans... ) would it be wrong to get them something off of their registry as a gift from us? I usually like to give a monetary gift rather than something off the registry, but truthfully this will be our like 6th wedding this summer and we have to travel and stay in a hotel and for us it would be cheaper to go the registry route. I guess my question is... is this appropriate, inappropriate... dare I say tacky???
TIA