Hi epaulino4
Are you sure it was Aerocity? I know that BW Carlos V have their own arrangement with a minibus service and if you booked it through them that would explain it. It happened to me once
I thought I was going on an Aeorcity minibus, but I wasn't
(the price difference was nowhere near as massive as the one you experienced though).
I have used Aeorcity on several occasions and found them to be 100% reliable and they charged only the amount quoted on their website. I have pre-booked previously too and the prices always matched up.
Just a thought.
Jo