If the basecamp app has a link you can click that says "go to this website to pay for basecamp", it's not allowed. You can sign in and purchase elsewhere, then

If the basecamp app has a link you can click that says "go to this website to pay for basecamp", it's not allowed. You can sign in and purchase elsewhere, then get the credentials and login on the app. It's fine, countless apps are like that. But if you do offer purchases with the app, Apple requires that you use the in-app purchase mechanism.