Amazon apply promo code post order?

Here's the situation: I ordered a micro sd adapter from Amazon last month, May 13. The estimated delivery was to be within the first 10 days or so of June. I got an email a couple days ago stating it would be pushed to late June (25-30). I just so happened to check on it on Amazon, in the invoice it said I applied a promo, however I remember (at least I think I do) unchecking the box which prompts me to apply the promo I have. I contacted amazon twice. The first guy was dismissive, despite telling me that there was a code applied for the item being delayed, then offered to cancel the order so I can get my promo back, I say that the item is more expensive and I would have to wait longer than I already have for it to be delivered/shipped. I contacted them again and the rep was more helpful, re-issuing me the promo code.

Question when you order something from Amazon don't they send you an email stating your method of payment, i.e. if you used a gift card or promo code? When I was looking through my emails I only had one order in which I used promo cred which was in 2010 and they included that info. Using that as reference, the order of the adapter does not state that I paid using any form of credit (promo or gift), which I didn't.

What it comes down to is: was I right in assuming that I did not apply the promo cred to my order and was right in contacting them asking for a re-issue/for them to come correct (referring to my memory and the email of the order), or did I use the promo cred and forget that I did (which is unlikely, I think)?

Title should be "Can Amazon apply promo codes post order", but I don't know how to edit titles  .