It means that by authorising use of your card (or paypal) to pay for the order you agree that the amount you pay to the seller might be different than the amount you see at checkout for various reasons.
It will normally tell you the highest amount you are authorising them to take from your card on the last page where you click the ‘Pay Now’ button.
I’ve never had an amount be different to what I thought I was paying but that’s not to say it never happens.
