Upsell.com ReConvert Bundles

It is a really interesting app, allowing you to meddle with the thank you page and add recommendations, pop ups and smart triggers to show different offers to different people. All in all it is better to have it than not to. And it doesn't cost a fortune. Furthermore, customer help has always been stellar. Vladimir and the rest of the crew have always stepped in swiftly to help me out with everything. :)