I am going to try the same thing with coke 16.9 oz. and pepsi 24 Oz. I am not worried about being denied a sale. Due to the fact that sams sells the 16.9 ounce Coke and 24 ounce pepsi, so you can buy as much as you like. What machines are vending the 24 ounce pepsi's . Do you need a glass front ?
The reason I was buying the 24 oz. bottles from WalMart is because my local Sam's Club does not offer them, though other larger Sam's Clubs do. If your's does, then fine - you shouldn't have anything to worry about.
I put the 24 oz. bottles in my BC10 soda machines. Those are made by Wittern Group (probably Fawn Manufacturing which is part of the Wittern Group). Sometimes they are called CB500s, depending on who you go through to get them.