Hi
My deal was this way I booked a car through RentalCars.com once you pay for the car they send you a Rental Voucher, I then went to alamo.com and used the online check in with the confirmation number on the Renatlcars voucher, this gets you a online check in confirmation number (xxxxxxxxxx) you then use this number to do the skip the counter deal. You need to decided a few things like extras and have your driving license ready , once done you get a Skip the Counter Ticket to print off which give you instructions on how to proceed once you are there.
From memory I think the guy in the booth at the garage exit, once you've picked up your car, needs your Skip the counter ticket, a driving licence, passport and the credit card you paid with and I think they check if you want to pay for fuel or bring back full. I don't think they hassle you for any other extras.
Then you drive off thinking how did I get away with that, with a happy DW and 2xDD
Regards
Paul