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What you are referring to is a "surcharge". Below is a snippet from the UK User Agreement:
5.4 PayPal and your customers.
In representations or in public communications to your customers, you shall not mischaracterise or disparage PayPal as a payment method.
If you enable your customers to pay you with PayPal, you shall treat PayPal’s payment mark at least at par with other payment methods offered.
PayPal does not encourage surcharging because it is a commercial practice that can penalize the consumer and create unnecessary confusion, friction and abandonment at checkout. If you are a User resident in the UK, you shall not surcharge for the use of PayPal. Otherwise you agree that you will only surcharge for the use of PayPal in compliance with any law applicable to you and not in excess of the surcharges that you apply for the use of other payment methods. You further agree that if you do surcharge a buyer, you, and not PayPal, will inform the buyer of the requested charge. PayPal has no liability to any buyer where you have failed to inform the buyer of any surcharge. You acknowledge that you could be committing a criminal offense if you surcharge and/or fail to disclose any form of surcharge to a buyer.
If you are a User resident in the UK and your Account has Merchant rate status and, at any of your points of sale (in whatever form), you:
PayPal may permanently downgrade your Account to the Standard rate (without prejudice to any other rights and remedies PayPal may have).
It's appears a UK seller or merchant can include the additional amount as long as they are upfront with you about their policies and you agree with their policies before completing your purchase.
Per the US User Agreement - surcharges are not permitted however, the seller or merchant can include a "handling" fee - for example, some ticket sales businesses do this a part of their daily business regardless if the sale is via PayPal or any other other method.