IAP Options and Pricing

Hello Friends,

I will be integrating some IAP features in my app and had a few questions regarding them

(1) I am aware that android and ios charge 30% of the the amount, but if I use a payment gateway such as paypal or google wallet will there be any other charge per transaction?

(2) What are some of the cost effective and also easy to use payment gateway options?

(3) Will there be any additional charge on the users who buy the IAP features?

Please let me know guys. Thank You

Arra