Thanks for your reply.
You're right but that's the only way I can think of when it comes to cover for payment gateway charges since WHMCS did not have a feature for transaction surcharges. Even if they have, I think it against Paypal t&c.
I really think that it is unfair for payment gateway to have a percentage charges. It should only be a fix charges. They are not share holder...
Anyway, instead of making a hook I think of setting 1 new product and name it Add Fund, under group which have all payment method like Paypal, credit/debit card. When a customer add fund using this method, after receiving their payment, then manually add fund to customer account with the amount after the transaction charges.
For all product, at the order form, will show only the payment method without transaction fee, such as Bank Transfer for domestic customer.
This way I think I can offer them to add fund or to pay with my selection of payment method with no transaction charges.
Is it possible to do this? Gonna try it anyway.