So I just want to make one thing clear: yes, cashiers do have to sell the upgraded memberships (the plus memberships) at the register - its annoying but thats our job. There does seem to be some confusion about the various programs that Sam's Club offers that offers cash rewards.
First program: the plus membership. You get 5% cash back on everything except milk, alcohol, tobacco, gift card purchases, etc. So in other words, for every $500 you spend, you get $10 loaded onto your membership card. This accumulates over the course of the remainder of your MEMBERSHIP YEAR. So, if you bought the upgrade for $55, you have a full year to make that money back. If you bought it for $27, you have 6 months and so on. Forty-five days before your membership expires the money will become available ON THE REGISTER for you to use to pay for the renewal, pay off groceries, or if you go to the membership desk, you can walk out with cash in your pocket. YOU WILL NOT GET A CHECK!!! I Repeat: You will NOT get a check.
Second Program: Sam's Club master card. You get 5% cash back on gas, 3% on dining and restaurants, and 1% on everything else EXCEPT milk, alcohol, tobacco, gift cards purchases, etc. There's no annual fee and sometimes there are promotions that Sam's Club does if you are approved for the card; either $20 off a purchase of $45 or $40 off a purchase of $100. Once a year, you will get a check in the mail.
Note: THE PROGRAMS ARE COMPLETELY SEPARATE. This means that a cashier cannot look up how much you have in rewards on the credit card and the credit card company cannot look up how much you have in cash rewards for your plus membership.
TL;DR: The plus membership and the credit card program are completely separate programs.