Pepsi would love to have some more details, so that they can check their quality issues.

Look at the end flap of the box, right below the "Best Taste Use By Date". There's a long number(there may be letters, too. I didn't look that closely at my Diet Pepsi 12-pack.)

Write again and provide this number. It will tell Pepsi when and where your 12-pack was made.

Also let them know the approximate date and place you purchased the product. There may be a problem with the way the shipper or the store handles the product.

You could also take the box back to the store. You might have trouble getting them to refund the price, though, since it seems that you tested each can. There's no product for the store to check. I have no doubt that you're telling the truth, but sadly, some people will lie.

When you provide Pepsi with a little more info about the flat product, they'll likely send you a coupon for a free 12-pack or your money back. Probably not both.