What you have in mind is called a "surcharge" which is not permitted as stated in the User Agreement. But, there is a work around - it's called a handling fee. If using PayPal's item button code, you would need to add the "handling" variable to the code.
Here's a snippet from the User Agreement concerning Surcharges:
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.
