UpSellit is the more common of the two in our index: it runs on 54 websites against 5 for CartStack, and 1 sites use both. Counts come from real scans, not vendor claims.
CartStack is a SaaS solution that allows any company with an ecommerce site or reservation system to increase revenue through reminding/encouraging consumers to return to their abandoned cart and complete their purchase.
UpSellit is a performance based lead and cart abandonment recovery solutions.
1 site runs both CartStack and UpSellit. That is 20% of the sites using CartStack, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.