Paypal AGAIN

Paypal overcharged me, not that much and (just) for the second time, but I really don't get it.

They tell you the shops are responsible, like they transmit the payment information.

But you enter their! site with a certain amount, you see a certain sum in their! payment interface when you hit their! pay now button.

And they charge you for a different amount afterwards.

How is that even possible, the initinal amount should be locked (for a short time), or at least you should see a warning when the amount has changed in the background.

It's like signing a contract and while the pen is just an inch away from the paper the contract is being exchanged unnoticed....

*Rant mode off*