Alamo used to have a Walmart code, and that code was published on Mousesavers. However, it was confirmed about a year or so ago that Alamo no longer has a code for Walmart customers. Mousesavers removed it from their site as it is no longer a public use code.
If you are using that code and it was accepted, it would be considered suspect and not a code that you may be eligible to use.
You can stack a contract ID with a coupon. I have stacked a contract ID from one place with a coupon from another (ex. Costco and Entertainment).