koltunowicz, the EXACT same thing happened to us for our December rental. (I posted about it in another thread.) The price changes if you try to view, modify, or check in the reservation. The higher amount also shows in the kioske when you check in at the airport rental center. You have to go to customer service, which is upstairs where the cars are, and show them your original reservation confirmation or e-mail. For us, they adjusted the price back to the lower, original e-mail amount.
I have been in touch with Alamo about this, and at that time they had no clue about it. By now they should have heard from other people who have the same problem. My current theory is that we both used coupons or an affilliate code. You used ACG and I used Costco. I did not change anything like a date or times or driver's license.
You might send them feedback on their web site. There is an agent named Jerry Aufdenkamp who helped me.
I haven't tested it recently because Alamo's rates are so high (over $400 for a week for a full). Instead I booked a Dollar Full size for $202. I haven't used Dollar for years, but it was better than using Alamo's terrible new web site and higher rates.
- bcc