Re: The problem is there's no defined standard, so it's roll-your-own (again)
"In order to not be challenged it would need to be digitally signed"
Nonsense. Most physical receipts are not tamperproof, so why would this suddenly be a requirement for digital receipts? If the retailer disputes the transaction, their financial records will show whether the sale took place. And if the payment wasn't in cash, it will be easy for the customer to show that a transaction did take place; this sufficient proof of purchase - irrespective of the availability of any receipt - for any statutory claims against the retailer.
Of course, for unwanted items and other 'goodwill' claims, the retailer can define any conditions they wish; the consumer has no statutory rights in this instance.