Add the meal separately, run the details in as they would be if they’d be getting the meal deal upgrade but keep it as a single.

example:

Single Big Mac

Medium Fries

Medium Coke Zero

—

Meal Deal Upgrade £0.00

- Large £1.00

”view quantity” checks if the item has already been redeemed, select it and if there’s no error message — continue to choose the meal deal size