we need to have a 50% deposit or Pay in Full option. It needs to be in the cart and not a coupon. Customer will select the options they want and I would like to have the 50% off this total after they select.

Looks like it will have to be a coupon.

It can be done with some custom coding - and just in the cart.php page without modifying the core files.
AI models are great at writing if you can tell it what you want. Claude and Chat GPT (Codex) are both great.

You can literally download codex on your pc, show it the folder where your site is and tell it want you want and it will write all the code for you.
In a few days, I had codex write my new plaque designer - see it here - https://www.trophykitsdev.com/dev5/proddetail.asp?prod=PLQ-P
No mods at all to the vsadmin files.
Again, you need to be able to give it some prompts of what you want to get best results.
If you want to explore that option, let me know and I will see if I can get you going with it.
I have team viewer so I can remote into your pc and show you how to do it.
You would need an subscription to chat GPT, but it's is very reasonable.

NOTE to anyone reading this - Regarding AI coding.
There is no way I would consider having it code a shopping cart like ECT. While it could probably be done, there are many security concerns with a shopping cart that AI coding might not put into place unless you know what to ask for.. Not to mention the ECT platform is built to accommodate a number of different database types and server configurations.

However, AI is overall great and fast with coding and should be a tool in your toolbox.
Many of the custom mods asked for here and easily be achieved with AI and no mods to the core files.

Thanks,
David