Well... It doesn't really make sense in-game. I mean, in Ishgard you're renting passenger space on some NPC's airship, and the vouchers are pretty much your tickets to pay the NPC back for having fuelled his ship up. But for the FC airship explorations, you're going out with your own airship, so why would you pay yourself with a ticket/voucher instead of just getting the fuel straight up? ^^;
