The online offer is strictly to see who is interested in volunteering. First off, it is not always used. Second, if you are selected, my experience has been that everyone being VDBed was given the same voucher value, regardless of what they bid. The bids, I have heard, are also used to "rank" volunteers in a given order. I typically bid about $5 less than the normal amount I would expect to receive, if I am truly interested. If they can get volunteers at the gate to take less than your bid, then those persons get the VDB. Remember, YMMV.