I think that the invoice shows the pre-order date to prove entitlement to the pre-order offer - in my case a keyboard for my tab S8. I am keeping the Samsung 'order completed' email that came a few hours after delivery to confirm delivery date. Also the 'Thanks for your order ' email clearly shows that delivery will be days after the order. I don't see a problem in proving that 'purchase date' as required by the warranty' is delivery date rather than order date.
