You can't use the Ebay label and upgrade.
As you advertise free delivery, you could pay for a next day service at the Post Office then apply to Ebay for a refund on the Simple Delivery label. The only guaranteed next day service is Special Delivery which is pricey.
You can't charge the buyer through Ebay for the extra postage.
You do have Express Delivery on your listings for an extra charge - it's a shame the buyer didn't choose that and avoid this messing around.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)