Moises,

The Alamo codes you mention are specifically for non-US travel industry staff as they include all the collision damage waiver and extended protection charges as standard. I've been using them for the last 15 years or so.
There are two codes to use :
'W9' is the standard 'fully inclusive' rate that covers all the extra insurance type waivers you now need in the US ( which cost a silly amount of money if you have to pay for them separately )
'RP' is the discouned 'Alamo Gold' rate which includes the above, up to 3 extra driver and a tank full of fuel
( the RP rate is well worth using if you have several drivers as its normally USD 9 a day each also you HAVE to pay for a full tank of fuel if you use the W9 rate so take that into account when calculatingthe price differential ).