[mcv]

You won't get charged twice on any well-written site, but I'm still careful with the back button around any transaction.

It's not really an issue at either of my banks, because a transaction is a lot more than merely submitting a form, and requires authorization with a single-use code. But webshops tend to have far more linear transactions, and I don't always fully trust them to use the back button the way I think it should work.