The first time I tried this I had 2 Vanilla bottles and asked the cashier to do a price check. The first one rang up 74¢, but the 2nd bottle rang up $4.99 so I took it off. After I paid, I asked her to try the Vanilla bottle again...and it rang up 74¢ (the 50¢ ZVR came off again...weird).
Yesterday (after I reloaded the ZVR) I tried the Orange. It was 74¢, and again we tried a 2nd bottle but it was full price, so I took it off. I did a second transaction with the 2nd bottle and it rang up $1.24 (no additional ZVR like yesterday).
So I think we have to buy these 1 transaction at a time.