Here are well-regarded Shopify apps for upselling and cross‑selling (short summary, who they’re best for, quick pricing note). I included official app pages so you can compare features & trials.

1. One Click Upsell — Zipify OCU

• What it does: Pre‑ and post‑purchase one‑click upsells, slide‑cart upsells, AI suggestions, funnel/A‑B testing.
• Best for: Scaling stores and Shopify Plus merchants who want advanced funnel control.
• Pricing note: Free/dev tier; paid plan scales with upsell revenue (starts around ~$8/month development tier; 30‑day free trial). (apps.shopify.com)

2. ReConvert — Upsell & Cross Sell

• What it does: Thank‑you / post‑purchase funnels, checkout blocks, one‑click upsells, A/B testing, analytics and segmentation.
• Best for: Small → mid stores that want easy post‑purchase funnels and lots of built‑in templates.
• Pricing note: Freemium tiers (paid plans start very low, e.g., $4.99–$14.99/month tiers depending on usage). (apps.shopify.com, reconvert.io)

3. Honeycomb (Conversion Bear) — Upsell Funnels

• What it does: AI recommendations, product/cart/thank‑you/post‑purchase upsells, funnel builder and split testing.
• Best for: Merchants who want fast setup + visual funnels and strong support.
• Pricing note: Free tier with limited monthly views; paid tiers from ~$49.99/month. (apps.shopify.com, conversionbear.com)

4. Candy Rack — All‑in‑One Upsell

• What it does: Embedded one‑click offers on product pages, cart, checkout and post‑purchase; AI mode and very fast performance.
• Best for: Stores wanting a lightweight, native‑feeling upsell experience with tiered pricing by store order volume.
• Pricing note: Free/dev; paid plans roughly $39.99–$79.99/month depending on order volume. (apps.shopify.com)

5. Frequently Bought Together — Code Black Belt

• What it does: “Frequently bought together” style bundles, automatic AI recommendations plus manual bundle control and discounts.
• Best for: Merchants who want classic FBT widgets and easy bundle discounts.
• Pricing note: Starts around $19.99/month. (web.codeblackbelt.com)

6. LimeSpot Personalizer (AI Bundles & Upsells)

• What it does: AI‑driven recommendations, bundles, frequently‑bought‑together, checkout & post‑purchase upsells, personalization across pages.
• Best for: Merchants with larger catalogs who want AI personalization and deeper analytics.
• Pricing note: Free tier available; paid plans for advanced features (multiple pricing tiers). (apps.shopify.com, limespot.com)

7. Bold Upsell — Bold Commerce

• What it does: Smart offer recommendations, subscribe‑and‑save upsells, works across product/cart/checkout and integrates with Bold’s subscriptions and other tools.
• Best for: Merchants seeking a mature enterprise solution with built‑for‑Shopify certification.
• Pricing note: Offers free trial; commercial pricing tiers for growth and enterprise. (boldcommerce.com)

8. Upsell & Cross Sell — Selleasy (Logbase)

• What it does: Pre‑ and post‑purchase cross‑sells, frequently‑bought‑together, bundle offers, with order‑based pricing tiers.
• Best for: Small→mid stores that want a budget‑friendly, high‑reviewed app with easy setup.
• Pricing note: Freemium (free up to a low order count), paid tiers from ~$8.99/month upward by order volume. (apps.shopify.com, logbase.io)

Quick selection checklist (pick the app that matches these priorities)

• Need post‑purchase funnels or one‑click post‑purchase offers? Prioritize Zipify OCU, ReConvert, Honeycomb or LimeSpot. (apps.shopify.com)
• On Shopify Plus and want native in‑checkout offers? Check Zipify OCU and LimeSpot capabilities and Plus compatibility. (apps.shopify.com, limespot.com)
• Limited budget / small store? Selleasy, ReConvert and Candy Rack have tiered/freemium plans that scale with orders. (apps.shopify.com)
• Large catalog / personalization priority? LimeSpot or Bold for AI personalization and advanced segmentation. (apps.shopify.com, boldcommerce.com)

Quick implementation tips (best practices)

• Start with one placement (e.g., cart or thank‑you page) and run an A/B test to measure AOV change.
• Keep offers simple and relevant (complementary accessories or small add‑ons convert best).
• Limit friction: prefer one‑click adds or post‑purchase offers that don’t require re‑entering payment info.
• Track metrics: AOV, take rate (% of orders accepting upsell), and impact on conversion/return rates.
• Watch site performance and mobile UX—choose Built‑for‑Shopify apps when possible for speed. (apps.shopify.com)

If you want, I can:

• Narrow these to the top 2–3 that fit your store size and traffic pattern (tell me annual revenue or monthly orders), or
• Compare pricing/features side‑by‑side for 2 apps you’re deciding between.

(If you prefer, install links are on each app’s Shopify App Store page—I cited those pages above.)