It's shouldn't really affect customers in any way. By terms of Express Checkout, after returning from the authorization at PayPal (which is NOT a checkout screen), the business must show the final checkout screen with the finalised price. If the business doesn't do this, it is the business committing fraud. PayPal is rather good at holding payments from businesses until they are happy everything is legitimate.
Maybe, but the implied behavior from what the buyer sees is that they are verifying a specific amount on Paypal's (trusted vendor) site, not on marginally trusted random internet vendor.
The difference is that if I want to order some two dollar bike parts, I'm happy to risk that I won't receive them, but I'm not in the habit of giving my credit card to every random site on the internet.
It doesn't really matter because the customer isn't on the line. It's PayPal that is. Considering how fanatically PayPal fights fraud, that they don't consider this an issue pretty much tells you that it isn't one.
Sure the customer is on the line. You make a very large charge on a customer's credit card, and best case is that they can't make further charges on the card because they've hit their limit. Getting it resolved in a week or so is little consolation when you have a useless credit card.