How to apply store credit manually on a draft order?
You can manually use the customer's store credit inbehave of the customer and send them the invoice checkout order directly using Shopify's draft order.
1- Go to Shopify admin > Orders > Drafts
2- Click “Create Order” button.
3- Click “Browse” and choose the order products
4- Enter the customer name in the Customer search field.
5- Click “Add discount” and enter the store credit amount value
6- Set the discount reason "Store credit" (Important)
7- Click “Save”
Please check this step-by-step video: