I'd love to, though my experience hasn't been super positive with this approach. I've tried this on the App Store. You'd be surprised how many support queries, feature requests, negative App Store reviews, etc developers get from folks installing the app for free who never intend to upgrade/unlock.

edit: It's not uncommon to try apps on the App Store and request a refund if it doesn't work out https://support.apple.com/en-us/118223