The Cart upsell shows recommended products next to what the shopper has already put in their cart. It is the last easy add before checkout starts, and it converts because the shopper has already decided to buy.
A cart drawer with the shopper's items at the top and a CartSella row of two recommended products with Add buttons below

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Where the shopper sees it

Inside the cart, wherever you place the block on your cart template. On most themes that is the cart drawer that slides in from the side, and the cart page, since both render from the same template.

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Set one up

1

Create it

Upsells → Create upsell → Cart upsell.
2

Name it

Fill in the Storefront title. Add a start and end date if it is seasonal.
3

Choose when to show

Always, above a cart total, or only when a specific product is in the cart. See below.
4

Pick the products

Manual or Auto.
5

Save and copy the ID

The sidebar shows the Upsell ID once saved. Drafts have no ID and never reach the storefront.
6

Add the block to your cart template

Click Add to theme in the sidebar, or go to Online Store → Themes → Customize, switch to the Cart template, click Add block, pick CartSella — Upsell under Apps, paste the ID, and click Save.
The CartSella — Upsell block added to the Cart template in the Shopify theme editor with the Upsell ID field filled in
It is the same CartSella — Upsell block used by the Products Upsell. The only difference is the template you put it on. The block reads the ID and renders the cart layout for a cart upsell.
Put the block on the cart template, not the product template. On the product template the same ID renders as a product-page recommendation row, which is not what you configured.

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Block settings, in the theme editor

Some themes make it hard to drop a block inside the drawer. The editor sidebar has an Embed anywhere snippet, a one-line piece of HTML you can paste into any part of your theme, and the recommendations render exactly there.

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When to show

“When cart reaches an amount” pairs well with a free shipping threshold. Show the recommendations to shoppers who are already close, and let the cart progress bar do the nudging.

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Products to recommend

Hand-pick the products or variants. Predictable, and the right choice for a small catalogue or a specific add-on you want in every cart.
By default a Cart upsell has Exclude products already in cart switched on, so it never recommends something the shopper is already buying. You can turn it off in Advanced options.

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Everything in the editor

The Cart upsell editor showing the When to show, Products to recommend and Cart drawer appearance sections with the sidebar preview

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Eligibility & targeting

Click Add rule for Traffic source, Customer tags, Customer location or Markets (the last only appears when your store has more than one market).
Customer tag rules only filter shoppers who are logged in. A guest is never hidden by them. Location and market rules always apply. If a rule is set but the storefront cannot check it at all, the block hides itself rather than showing the wrong thing.

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Advanced options

  • Priority, 1 to 10, default 5. Lower wins when two cart upsells qualify at the same time.
  • Maximum uses, total and per customer. Off by default. See Advanced offer settings.
  • Exclude products already in cart, on by default here, with Match recommended product variants as a sub-option.
  • Offer subscribe & save, adds a “Subscribe & save” toggle to any recommended product with a Shopify selling plan.

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Discount

You can fill in a discount, but the recommendation appears at its normal price. CartSella does not advertise a discount the checkout will not honour. For a real discount on these products, create an offer and let the upsell recommend the product.

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How it looks

Two places control the look, and it is worth knowing which is which. Per upsell, in the Cart drawer appearance section of the editor:
  • Visible products: 1, 2 or 3 cards.
  • Buttons: show the Add to cart button, show the View product link, or both.
  • Section title: the heading above the recommendations.
  • What to show: product description, compare-at price, discount badge.
  • Card style: gap, border, padding, corner radii, shadow, hover scale, image aspect ratio.
  • Text sizes and Colours, inheriting your theme until you switch that off.
Globally, on the Customize page under Cart upsell, for the style that should be the same on every cart upsell.
Once you save a global style for the Cart upsell, it takes over from the per-upsell appearance for every cart upsell on the store. Pick one place and stay there. The visible product count and the button toggles always stay on the upsell.

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Limits and gotchas

  • Drafts never render. Set the status to Active and save.
  • The CartSella app embed must be on. See Storefront blocks.
  • Some themes rebuild the drawer contents on every cart change. If the recommendations flicker or vanish after an add, check Troubleshooting.
  • Run one cart upsell at a time. Two competing in the same drawer is noise, and only the lowest priority number shows anyway.
  • Keep it to two or three cards. The drawer is small, and the checkout button has to stay in view.