Upcart—Cart Drawer Cart Upsell

I would rate it 6 stars if I could, not only does it do exactly what it claims to do but the team over at BEAM has been incredibly helpful in implementing and changing the app as needed. The functionality of this app is fantastic, and is very approachable while at the same time being very extensible and able to be tweaked to your heart's content. I HIGHLY recommend this software, as it works great.