We are currently creating automatically draft orders using the new Draft Order API (which is awesome, by the way) and send the invoice URL by email to the customer. However, it happens that when the customer click on the invoice_url link, the product may have become out of stock. Depending on how the stock is configured on the merchant's store, they may prevent the product to be bought. The invoice URL therefore displays an error saying that the product is out of stock.
The issue is that in the page, the customer has no way to simply remove the products that are no longer in stock (he may be interested in the other products in the draft order). The only solution is to have the merchant manually update the draft order to remove the out of stock lines, but this is definitely not really convenient.
Are there planed improvements to make this more flexible ? :)
At the moment, this is expected behaviour. Since Draft orders do not have carts associated with them, there's no method to alter the items included in the checkout. There are no current plans to add some way to alter which products are in an invoice. Either a merchant would need to send a new invoice, or the customer would need to create a cart through the storefront.