Unless you’ve never bought anything online, you’ve already given your email address to a host of retailers, so I can’t see how declining to do so at a till is particularly keeping you safe from the hypothetical data breach.
My point was generally Google has just made it easier for retailers to get their receipts into their customers’ banking apps, making Flux worthless to most shops, with the benefit that they also get the email address data first hand, and don’t have to share it with a third party. The retailers will make the decision for you, by just not taking up Flux.
This is probably a discussion for the Flux thread.