Pass PayPal transaction fees onto the customer

This should be built into your package pricing, for example passing the paypal fee on directly to your customers is against PayPals user agreement. Many credit card processing companies also have these terms. So just do what everyone else does and build it in to your package pricing. (And it only cost a little bit of thing, no $$)

This would be a violation of PayPal's User Agreement - see below:

1. 4.5 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.

Reference: https://www.paypal.com/webapps/mpp/ua/useragreement-full?country.x=US&locale.x=en_US#4

Most people include the extra little bit of pricing in the product pricing by default, this removes the violation as you are charging as the product pricing and not a super fee.

The best way would be to add a little bit to the product pricing as Thomas said, this removes the violation as it is classed as the product pricing and not a super charge, it is the same price across the board, no matter what payment gateway.

Paypal allows that a "handling fee" may be applied. Therefore, if you charge 4% handling fee it's OK. A handling fee is the same as an admin fee. Accepting PayPal does incur more administration, especially in terms of balancing your books. Handling Paypals peculiar methods of accounting can be very time-consumming.