The most confusing thing of shopping at Walgreens is knowing when you can roll Register Rewards - use one and receive another one, but it is really easy once you know what to look for.
On a Register Reward, below the Walgreens logo it will tell which retailer offered the reward. Check the packaging of every item you are purchasing. If it is a different manufacturer/retailer, you will receive another Register Reward.
It's not uncommon for a cashier to tell you that you won't receive another Register Reward if you use one. I always ask if this a new rule within the last few days. You can see the cashier mentally searching to remember when they were told that by their manager.
At this point, I ask them ring up my purchase and see what happens. If the cashier seems amazed and interested that more Register Rewards printed, I explain the rules - obviously their manager didn't. If the cashier could care less, I take my new Register Rewards and go on with my business.
