Here are popular, well-reviewed Shopify apps for upselling and cross‑selling, grouped by best fit so you can pick quickly. (All apps offer free trials or free tiers — check the app listings for current plans and limits.)

Top picks by use-case

• Post-purchase / funnel-style one‑click upsells: Zipify OneClickUpsell (OCU) — best for building multi‑step pre/post‑purchase funnels and A/B testing; supports in‑checkout offers for Plus stores. (apps.shopify.com)
• Checkout & thank‑you page upsells / full checkout customization: ReConvert — focused on thank‑you/post‑purchase funnels, checkout upsells (Shopify Plus where required), drag‑drop editor and analytics. (apps.shopify.com)
• Sitewide AI recommendations & bundles: LimeSpot — AI product recommendations, “frequently bought together” and bundle offers, personalization and A/B testing. Good if you want automated personalization across pages. (apps.shopify.com)
• Amazon‑style product bundles / frequently‑bought‑together: Frequently Bought Together (Code Black Belt) — simple, high‑conversion bundle widget and one‑click add‑to‑cart for cross‑sell bundles. (apps.shopify.com, web.codeblackbelt.com)
• In‑cart / cart‑drawer upsells and conditional triggers: In Cart Upsell & Cross‑Sell (ICU) — strong for cart/cart‑drawer offers, conditional logic and AI autopilot. (apps.shopify.com)
• Lightweight post‑purchase funnels / revenue-share pricing: CartHook / similar post‑purchase apps — drag‑drop post‑purchase funnels and 1‑click offers (often use rev-share or tiered pricing). Good for stores that want pay‑for‑performance models. (apps.shopify.com)
• All‑around “AI smart upsell” apps: Bold Upsell (Bold) — supports product, cart, checkout and post‑purchase offers with AI smart offers and multiple placements. (apps.shopify.com)

Important platform notes

• In‑checkout upsells and deep checkout customizations frequently require Shopify Plus (or apps that use Shopify’s checkout extensibility for Plus stores). Many apps will show post‑purchase / thank‑you upsells for non‑Plus shops, but in‑checkout blocks are often Plus‑only. (Check each app’s listing/details for exact compatibility.) (apps.shopify.com)

How to choose (quick checklist)

• Goal: post‑purchase funnels vs. cart/product page recommendations vs. sitewide personalization. Pick an app that matches the placement you care about (product page, cart drawer, checkout, or thank‑you). (apps.shopify.com)
• Budget & pricing model: some apps charge flat monthly fees, others scale by orders/revenue or take a % of upsell revenue — compare expected ROI and free‑trial limits. (apps.shopify.com)
• Testing & analytics: choose apps with A/B testing and clear analytics if you want to optimize offers (Zipify, ReConvert, some LimeSpot/Bold plans include testing). (apps.shopify.com)
• Theme & mobile compatibility: ensure the app supports your theme and mobile/cart drawer layout — most top apps auto‑style but double‑check during trial. (apps.shopify.com)

Suggested next steps

1. Shortlist 2–3 apps that match your primary need (e.g., Zipify or ReConvert for funnels; Frequently Bought Together or LimeSpot for bundles/recommendations). (apps.shopify.com)
2. Install during a low‑traffic time, use the free trial, and run simple A/B tests (single‑offer vs. no‑offer) to measure AOV and conversion impact. (apps.shopify.com)
3. Watch for checkout/Plus requirements if you want in‑checkout offers; otherwise focus on cart and post‑purchase placements that work on all plans. (apps.shopify.com)

If you want, tell me:

• which Shopify plan you’re on (Basic, Shopify, Advanced, Plus), and
• whether you want pre‑purchase (product/cart) or post‑purchase (thank‑you/one‑click) upsells —

and I’ll give a 2–3 app shortlist and the exact features to test for your store.