Hi, @mostal. Thanks for your question, and welcome to the Community!
Customizations to the checkout, even with an app, are not possible unless you are on the Plus plan, I’m afraid. However, it IS possible to add a checkbox to the cart page of your online store!
We have some apps in the Shopify App Store for adding a checkbox with specific purposes, such as:
It’s also possible to add an ‘Agree to terms and conditions’ checkbox to your cart page through some code customization of your theme. If you prefer this option, and you are comfortable with coding, the steps to implement it are found in this Community post. If you would like assistance with making this change, please contact a Shopify Expert to assist.
Could you tell me more about what it is that you are wanting to have your customers confirm via the checkbox?