How do I add add an option to a product in the cart?

Store Design
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1

When I have a product in the shopping cart, I’d like to add an option to it. When existing options are rendered, they’re just printed out with a:

{%- for line_item in cart.items -%}

That renders list items that look like this:

- Age: 42

In the example

  • above, the customer entered their age as 42 on the product page and it was then displayed from cart.liquid.

    I would like to add another option to the cart from cart.liquid. I can’t simply add another

  • since that’s just what’s rendered and passed to the front-end, so after clicking Checkout it won’t be there anymore.

    Is it possible to update the cart from the liquid code here, perhaps with a script tag?