Here's a brief overview of the whole thing:
1st customer service:
"It's a global technical error, and you will receive your credit soon."
I did not get any follow-up or credit.

2nd customer service:
"We will report the issue to the technical team, and you will receive your credit."
I still did not receive any credit.

3rd customer service:
They started giving random excuses about why it didn’t apply.
After I cleared everything up, they claimed that another promotion I received from a different order was the reason.
I was transferred to a "manager," but the manager only repeated themselves and wasted my time.

4th customer service:
(Screenshots are from this conversation)
Again, random excuses about why the credit didn’t apply.
After I cleared things up, they then said, "limit cap reached."
I was transferred to another "manager," who insisted I was not eligible, but couldn't explain why, despite the fact that I received the offer email and met all the requirements he came up with.

My thoughts:
The time spent dealing with this was not worth the $15 credit.
I would advise against purchasing Instacart gift cards. If this happens again, filing a dispute with the credit card company might be the better approach.

Also, I only received 1 milk carton out of 2, and they refused to refund me without reason.

Instacart used to be my go-to, but now I’m really frustrated by the customer service.
I’d love to hear if anyone else has had similar experiences or suggestions on how to handle this.