Increasing In-app Purchases

My macOS app offers free download and trial, but requires an in-app purchase to unlock full functionality. Currently, only 2-3% of the downloads result in purchase. Presumably the rest either delete the app or keep it but don't use it. I'd like to target such people - maybe after a week or so after the initial download - with a special offer like a small discount on the purchase price. Any suggestions on how to do this?