This has been extracted from my personal library, you can view and copy the code from this page's custom code section.
You will also need to apply a custom attribute to the button you'd like a user to click to add multiple items to their cart at once.

The custom attribute is does="add-all-to-cart", you can see in the screenshot below how it has been applied to the 'Add All items To Cart' button above, you can also view this page in webflow and click on the button before pressing D to display the settings panel and view how it has been added there.
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The button will pick up all of the products that share the same direct parent element. In this example the direct parent of the button is the div with the flex-align-center class. The button will not work if it is inside of another div, its direct parent must be a parent of the collection list that contains the products. The products have to have the add to cart button for this to work, however it can be hidden from the user.
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