My app was rejected citing 11.3 (In-app purchase)
Read through the other points of the App Store Review Guidelines under Section 11. Purchasing and currencies. One more that stands out for your situation is this:


  11.13 Apps that link to external mechanisms for purchases or subscriptions to be used in the App, such as a "buy" button that goes to a web site to purchase a digital book, will be rejected


So while you could provide a link that will open in Safari (instead of opening the page in-app), if that purchase will be used in your app in some way, it's likely to get rejected by citing this clause.

Also keep in mind that any interpretation here or elsewhere of single line policy statements to provide a clear "yes" or "no" to questions may not be final. It's Apple's interpretation that actually matters.