ReConvert post purchase upsell

This app is super customizable. It does just about everything you need out and if there is something that isn't available out of the box, you can add blocks where you can drop custom HTML.

Added bonus that I didn't know when I signed up – you can actually add a discount code AND offer free shipping on an upsell product which is great (since no one wants to pay for shipping twice and normal Shopify discounts can't do this...)

The support is great as well. Helpful and responsive.