I'm experiencing a similar issue as what is being discussed. We are renting a house with another couple and splitting the cost. Final analysis: You can't pay balance early.
1. Made reservation and paid half for a house rental. The rest is due about three weeks before the rental date.
2. The other couple going with us on the trip wants to pay his half (the balance) with his credit card.
3. I added his card to my AirBB accout as the default.
4. I tried to find a method to pay remaining balance, then I would remove his card from my account. There is no method to do this. I
5. I called AirBB customer support and asked them to charge the remaining balance. The person helping me (well not really helping me) said that can't be done. There is not other way to pay the balance than the automatic payment which is scheduled in another week or so.
Some of you mentioned that the 2nd payment will automatically charge the original credit card even if you had changed the default. I asked them about this and they said the balance should come off of the "New Default" credit card. But they were not 100% sure. What I got from that phone call is they had no idea. This all seems so simple. I'd hate to see how they handle complicated things.
I will report back here next week if the "new" card works or if they still charge my old card.