Dynamatic Cart & Upsell Engine

Dynamatic is a great alternative to either the more expensive guys (Rebuy) or the ones that require multiple apps to cover all the bases (Upcart + Aftersell). Dynamatic does everything you need in terms of upselling and cross-selling. It has PDP widgets, a slide cart, post purchase checkout, and you can run campgaigns to switch things up during certain periods of time or for certain customer segments. It can be a bit complicated to set up initially but the customer support is helpful so don't let that deter you. Once you have the hang of it it's pretty easy.