Hi @Bill6 ,
If you are sending draft order invoice to customer manually, and open to using a workaround using an app, I think my app can help.
The default checkout page from the “send invoice” button does not allow customer to input discount code, my app workaround this by creating 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.
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.
After installing the app, go to your draft order, click “More actions” on top right > “Generate discountable checkout link” ,
Hope this can help.
Regards,
Axel Kee