Stores on general plans cannot edit the checkout experience , only Shopify Plus enterprise could edit the checkout.liquid before it’s depreciated.
To add functionality to parts of the checkout requires using or making an app.
This is a mindset that will limit you from using every tool available, stick to the time-investment, math and data.
There is an inflection point for adding such features as an app vs customizing to a theme.
If an apps cost in fee & time managing it is less than the revenue that app generates do you really want to spend time figuring out how to create a business feature that hasn’t even been validated yet with customers (read: shortlist and install “best” app to test conversions then invest). Customizations can cost easily cost 10x upfront vs the a monthly app cost.
/end caveat.
To do something like this on the /cart page , not the checkout, see the gift wrap customization. https://shopify.dev/tutorials/customize-theme-add-gift-wrap-option
See other conversations for auto adding products to the cart, and about hiding/preventing products from being added to the cart normally; though in typical fashion this is generally handles by apps like locksmith.
And there’s the backend problem of making sure orders(post checkout) are valid with total cart values to make sure customers aren’t being too clever by using the ajax api to add products to the cart.
Try and setup the free shopify-flow app to validate orders totals or an automation app like usemechanic to script custom order validation logic.
If you need this customizatio & process built you can contact me directly for services.
Please provide context, examples: post url(s), store url, theme name, or any further detail.
Contact Info in signature.
To restrict it to certain customers see conversations/tutorials on making a membership site with tags.