The current state of testing promotional codes in IOS is not developer friendly in the slightest. I have been trying to implement and test promo codes for a couple months now, while having the store kit integration helps slightly to mimic offer code redemption there is no friendly way of testing offer codes in test flight.
I have it implemented with presentCodeRedemptionSheet() it seems the easiest way to test the codes is release my app to production create a 3-day promo which is the minimum amount of days that can be set, then wait 3 days before I can test again in different ways.
One of my colleagues who is not a developer also wanted to test this feature and it did not work for her. Unfortunately it was a month long promo code she had redeemed so will have to wait till September to see if this is fixed for her now.
I have also must note the fact the code redemption sheet and setting up payments in apple all together is a buggy mess. Starting out I added my card via apple pay thinking this would be enough to go through the redeem code process. Then upon entering my code it would show the dialog to verify with touch ID along with my payment details and even a picture of my card. After verifying, it then kept showing a pop up that I must include payment information which I already had added via apple pay. In the end to fix this I had to remove the payment method restart the device, add the payment method again then restart the device again.
Another issue I bumped into is when you press redeem the "redeem offer" button is disabled but nothing happens for a while until it appears to timeout and you have to try again until it eventually works.
The final annoyance I experienced was creating the offer code for my subscription and immediately trying to redeem it apple said the offer expired even though i set it to the 31st of August and the date was the 2nd. Only way to fix this was once waited and keep trying banging your head against the wall until it eventually works.
In summary, I hope apple have a review of their offer code redemption process and make it more developer friendly and implement a way of testing within test flight whether it use live offer through test flight or include a new section to add test offer codes for subscriptions.
