You cannot do this. I run a website and mobile app ecosystem, which started as a website with standard credit card billing. If you want to mention that you have a website, or link to the website, you must provide every option the website has for purchasing, in your app, using apple's payment system. There is one exception, which is for physical goods/services, see Lyft and the like.

Sorry, but you have to take the 30% hit, or you can't mention your website, and you certainly can't put in a third party payment system. You'll get booted fast, or, won't even make it in the door when they review your app.