It's very simple, just visit https://app.upsellify.com/setup/install and follow the instructions.
The Upsellify app requires following permissions:
Firstly, select product and discount you want to offer on thank you page to your customers, and secondly, choose offer time limit and write short motivation text to make this offer even better. Then just sit down and watch incoming orders.
You can try Upsellify app for free for 7 days and then it will cost just 5 USD a month.
You can see terms and coditions at https://app.upsellify.com/information/terms.
Please feel free to contact us by email at firstname.lastname@example.org.
Copyright (c) 2019 Tomáš Halamíček