Hi Shopify store owners,
I have noticed quite some store owners wanting to let customer (or themselves) to add discount code to a draft order, but can't due to Shopify limitation . (eg: https://community.shopify.com/c/Shopify-Discussion/Draft-Orders-in-Admin-is-great-but-why-is-it-miss...)
Wanting to solve this, I have made an app that allows customer/merchant to input discount code at the checkout from draft order!
You can install the app (Draft Helper) here : https://apps.shopify.com/draft-helper , there's a free plan which you can try it out.
As the default checkout from the "send invoice" button does not allow customer to input discount code, but with this app, you can create a separate checkout from the draft order, and you can set a discount code on the checkout, or let customer to input it on the checkout. You can then send the link of this checkout to your customer.
After installing the app, go to your draft order, click "More actions" on top right > "Generate discountable checkout link", this will generate a checkout link which you can send to your customer, then they can apply the discount code there.
You can also add / edit line item properties for each product in the draft order, or rearrange the line items (don't need to recreate the draft order with specific sequence of products) using this app.
Hope this can help.