Great question — here are battle‑tested Shopify apps (grouped by tactic) that merchants commonly use to lift Average Order Value (AOV). For each app I give the quick use case and a source so you can read details and pricing.

Post‑purchase / one‑click upsells (very high ROI for many stores)

• Zipify OneClickUpsell (OCU) — industry standard for post‑purchase one‑click upsells and in‑funnel offers (great AOV lifts for stores and Shopify Plus). (apps.shopify.com, zipify.com)
• ReConvert — flexible thank‑you/thank‑you‑page/one‑click upsell builder (good if you want customized post‑purchase funnels & data capture). (reconvert.io)

On‑site upsells, cross‑sells & “Amazon style” recommendations

• Frequently Bought Together (Code Black Belt) — AI/manual bundle recommendations and single‑click bundle adds from the product page. (apps.shopify.com, web.codeblackbelt.com)
• PickyStory — product bundles, “shop the look”, and mix‑and‑match deals that convert product pages into higher‑value offers. (pickystory.com)

Bundle & mix‑and‑match builders (sell multiple SKUs as one product)

• Bundler — easy bundle creation, mix‑and‑match and quantity breaks. Good free/low‑cost option to test bundling. (apps.shopify.com)
• Rebuy (Bundle Builder / Personalization Engine) — personalization + powerful bundle builder for higher AOV (enterprise scale merchants use it). (rebuyengine.com, pickystory.com)

Quantity breaks / tiered pricing (encourage larger carts)

• Pareto / Quantity Breaks & Bundles (Quantity Breaks apps) or Discountly — let you set quantity tiers/volume discounts and BOGO to nudge customers to buy more. (apps.shopify.com)

Free‑shipping progress bars & urgency nudges

• Hextom / Free Shipping Bar (and similar announcement/progress bar apps) — show how much more a shopper needs to reach free shipping (simple, effective AOV lever). (ecommerce-platforms.com, owlmix.com)

Loyalty / referrals (increase order size over time)

• Smile.io — points, VIP tiers and referral programs that raise repeat purchase value and overall customer spend. (apps.shopify.com, smile.io)

Email/SMS personalization (increase AOV via flows & recommendations)

• Klaviyo — advanced segmentation, product recommendation blocks and post‑purchase flows to upsell and cross‑sell via email/SMS. (apps.shopify.com)

All‑in‑one CRO / upsell suites (if you prefer fewer apps)

• Varies by store; some merchants combine a CRO toolkit (progress bars, popups, cart upsells) with a dedicated upsell/bundle app. (If you want, I can suggest specific combos for your store.)

Quick recommendations for getting the best results

• Start with the biggest opportunity: if you don’t have post‑purchase offers, add a one‑click post‑purchase app (Zipify OCU or ReConvert) — these often produce big short‑term lifts. (apps.shopify.com, reconvert.io)
• Add product recommendations/bundles (Frequently Bought Together or PickyStory) to product & cart pages to capture incremental AOV on every session. (apps.shopify.com, pickystory.com)
• Use a free‑shipping progress bar and/or quantity breaks to nudge customers to cross thresholds. (ecommerce-platforms.com, apps.shopify.com)
• Always A/B test offers (price, copy, placement) and track AOV change + take rate for each upsell.

If you want, tell me:

• whether you’re on Shopify Basic vs Plus,
• number of SKUs (small catalog vs large), and
• what tactics you’ve already tried — and I’ll give a 1–2 step action plan (which app to install first + the exact offer to test).