Use the payment gateway webservice to build a in app payment sdk.
You must have built a IOS app that uses a mobile payment SDK.
Please give download link of the mobile app with payment integration you built.
Please answer questions below.
Which payment gateway have you integarted to a mobile app?
What was you greasted challege while intergrating that payment gateway?
What do you think the payment gateway provider should have done to make the intergration easier?