Just to make sure I follow - you've tried creating a draft order in the following screen:
And then when the invoice arrives at the customer's email:
You'd like then to be able to add some discount code. I've just added the link to the discount when I send then the notification, and then it shows up clearly in the email and they can use the code.
This is just a dev store, so no payment gateway == I cannot properly test it right now, but that should work.
If you're still stuck just @ mention me here and I'll take a closer look.
@stacie OK, so just to be perfectly clear about it - Shopify does not support allowing the customer to insert a discount into a checkout page page right now - see the link or the highlighted comment below:
Since this is not supported, I initially thought about cart attributes - see here: https://help.shopify.com/en/themes/customization/cart/get-more-information-with-cart-attributes?utm_... - but that will only help if they actually move through their cart before coming to the checkout (which they do not on the draft invoice flow you're looking for). Same goes for order notes and line order items, I think.
I think the reason this is not supported is because it's a feature for Shopify Plus - https://help.shopify.com/en/themes/development/layouts/checkout - where merchants in that tier can then edit this page directly, basically creating the flow of applying the discount code by hand.
I see two ways going forward: If you have Shopify Plus, then we can investigate further on how to customize your specific page so the order is discounted (if that's possible without your intervention on the backend, which I'm not sure it is). If you don't have Shopify Plus, then i would suggest asking the customer to leave the discount code in the order notes and re-imbursing him for the cost later (or some arrangement of this type).
If anyone has a better solution, I'd be really interested in hearing it. Sorry @stacie for not being able to help better!
Sure, no worries.
If you are a Shopify Plus merchant and would like to do some workaround there, ping me and I'll see what I can do.