My guess, based on the situation, would be that she meant "of course, it's fine, don't worry about it. Take your time", something like that. That's based on my hope that they wouldn't intentionally be really rude to you. However, it could also mean "Of course you didn't realise it, you idiot". Which, as I say, would not be very nice. It's all down to intonation and how they seem to be acting, so it's hard to tell from a written account. Don't forget that native speakers misjudge situations sometimes too. It's not always easier to interprete what someone meant, but I would like to give her the benefit of the doubt :)

i think 2. you're overthinking about it, but if you want a second opinion she's right, she works in a post office and you're new there and that's probably why you are nervous about making a good impression. Carry on Takuya and listen to Duncan, give her the benefit of the doubt. Truth is the holidays are coming and she's tired( everyone is) and wants to go home and relax. Stop apologizing unless you made a mistake on purpose or hurt that person in some way. Its human to make mistakes dude. Nobody expects perfection unless( they're your manager lol ) their pay depends on your performance. She tried to help you out because she pointed out the mistake to you in private, if she wanted to undermine you she would do it secretly to the manager btw. Focuss on the action, not the words. Was it helpful? In this case: yes.