I found the following Alamo codes in my research for an upcoming trip, they may help providing you are a member of them.
the BJ one you pointed out in your post offers only a 10% discount.

Corp. ID % Off Offered By
-------- ----- ----------
7014291 15.00 Amex My vacation 08 Selects channel
7014627 17.03 Costco Web Offers 09
7015309 17.03 Costco members

TS