check "accept the Terms and Conditions" on the client side

Supposing that you're using the new checkout, and supposing that you're clicking on the finish button, you want to edit the file show_block_buttons via the menu Display>Views and add your javascript check on the onclick attribute of the finish button.

Hi,

We had many issues with the old checkout system with the client side validation.
Most of the time, that was because other extensions or the template breaking the javascript on the page and because of that our javascript didn't work. Merchants were then asking us for help for something not comeing from our end.
Also, since there are many ways the checkout can be configured, the client side checks were quite messy and it didn't work well.
To avoid these issues, and because server side checks are necessary regardless of the client side checks, we focused on the server-side checks for the new checkout.
With the ajax system of the new checkout, the client-side checks are also less needed so we decided to not do them for now.
So far, everyone was happy with the change. We'll see. If we get more feedback on that, we might reintroduce part or all of them in the future.
Since the new checkout is still quite young, we need more feedbacks in the coming months before we can judge on whether that would be necessary.

The old/new checkout switch is controlled by the "checkout legacy" setting of the HikaShop configuration.