Here is the short, but necessary-to-have apps available at Shopify Store:
1) Upsell Motivator - Free Bar. You can offer "Free Shipping" or "Free Gift" on orders of certain amount and inform your clients about it using a beautiful pop-up bar.
2) Upsell Bundles. Create Bundles, Packages, Kits and Sets with discounts with a 1-click Add to Cart button and significantly increase the order size.