Unfortunately that is not permitted - what you have in mind is called a "surcharge" which is not permitted as outlined in the User Agreement.
Here's a snippet from the User Agreement:
No surcharges
You agree that you will not impose a surcharge or any other fee for accepting PayPal as a payment method. You may charge a handling fee in connection with the sale of goods or services as long as the handling fee does not operate as a surcharge and is not higher than the handling fee you charge for non-PayPal transactions.
Note, you can include a "handling fee" however, you should make sure that the buyer is aware of the additional amount and why. Just as an example, there are some ticket sales companies that will include a "handling fee" for processing your order or for other services. This is something they would routinely do for any purchase and is an advertised policy so the customers are aware of this up front.
