Actually last time I booked with a bunch of credits to pick from (I had 6) I figured out you can pick. The system automatically picks 2, but if you "deselect" one by clicking on it you can pick a different one (if I remember correctly the others were previously grayed out? Its been a few months, I don't completely remember). Thing is, once you deselect a credit it disappears from the list, kind of freaked me out at first, I thought it was gone for good. But after I finished the transaction I looked back in my account and the 4 unused ones were all still there. HTH