I can't say as I have ever had to pay to park at Walmart in Canada, but that is not the case in Guadalajara. However, it isn't just at Walmart (actually Walmart held off the longest with installing ticket meters). Most malls have a ticket meter (like you see above) at the entrance to their facility. After you shop you need to stop at a ticket machine to pay (just like at the Winnipeg airport) then you find an exit, insert your paid stub, and hope the arm goes up to let you out!
The 'out' at the mall
Now let me say, just so we are clear, it's not overly expensive; nothing like a parkade in Winnipeg would be. You've just got to remember to pay before you reach the gate as you CAN NOT pay there. The guy you see in the picture - he will send you back into the mall!