Technically you cannot do that as you would be applying a Surcharge. Per the definition of a Surcharge, this is NOT allowed as per the User Agreement. A savvy customer would file a dispute against you and win. Additionally PayPal could close or limit your account.
What's the work around, suggest charging a handling fee. Use the "handling" variable in the item button code. Ensure that you clearly explain that their is a handling fee applied to all transactions. To learn more about the use of this variable, check out this guide.
https://www.x.com/sites/default/files/pp_websitepaymentsstandard_integrationguide.pdf
