We sell bespoke furniture, and would prefer to take a small deposit on our existing shopify store, instead of the full amount. We use the Motion theme. If you're familiar with the shopify checkout, there is a column on the righthand side, where you can enter a discount code. I want to add a new function in this space, so there is also the option to pay in full, or to pay a deposit of £100.
It's important that if the customer adds a shipping method, or discount code (and therefore the price changes) that cost is still fixed at £100.
This option should only appear, or be allowed, if the total cost of the order is say above £1000.
I have drawn the idea on the attached screenshot.
Finally, we need the total cost of the order to appear in our Orders. And on the customer email.