Ugh. Delinquent Amazon store card payment.

I have an Amazon store card that I've purchased several thousand dollars worth of stuff on over the last couple years. I've always paid it off in a timely manner and never had any issues, and the last few months I had (what I thought) was a balance of zero. My girlfriend also has an Amazon account and sometimes uses it to buy on-demand TV shows and movies.

There was a slight misunderstanding where I had logged on to our Smart TV to purchase something and it saved my information instead of hers. Some time later she decided to buy some new shows and, after checking my email, I noticed it was being charged to my account. No big deal. We just swap over to her account and she pays me back.

After swapping over I went ahead and paid off my balance and didn't think any more about it. I have since not purchased anything on my store card so I thought there was no reason to check it (big mistake). It turns out one of the shows was charged to my account after I paid it off, resulting in $6.48 sitting in my account. This was back in September, so this amount sat in my account unpaid and accruing late fees for nearly 2 months.

I received my monthly Amazon card balance email stating that I owed 15 dollars or something like that. No mention of something being late or anything. I assume maybe my GF bought something and just log on and pay it off. It states the bill is due 11/20 so I just set it to pay off on that date.

Well anyways, I check Credit Karma today and coincidentally found out that as of today I also have a delinquent report from Amazon, resulting in my credit score dropping a whopping 75 points!

I am obviously freaking out and I call Amazon to explain my situation. I pay what was owed on the phone and the gentleman I talk to explains that the delinquency will be removed and not to worry about it. He mentioned that he saw me initiate the payment a couple weeks ago (not knowing it was late) and that I've been on time with all my other payments. The problem is that I AM worried about it! I've worked hard to get my credit score where it is and I'd hate to ruin it from one mistake. Am I overreacting and have I done everything I need to, and this bad experience will be erased from my credit report soon?

FWIW I called again and spoke to another guy (both Indian with heavy accents so it is hard to understand them) and I told him I would like an email or something stating that they will remove. He (the best I could understand) said that it will be removed and to call back next month if it is still showing up. I'm still worried.