NCL was having problems with their app earlier this year, pretty sure there were several discussions about it. I had a cruise in mid-Feb and at one point got a balance-due message, for the amount of my DSC. I had planned to pay it in advance anyway, so paid it and they refunded it - I hadn't actually indicated what I was paying for. But every time I went to my account during that time, I got a link to pay my balance, which showed as $0.
Recalled email??? Not really a thing as far as I know, at least as far as sending external mail. I heard recently that recalling emails might be coming on some platforms but not currently available. Attachments are stored on my computer as soon as they are downloaded - and as pdf, not a link to NCL.
So who knows what really happened. In my experience, if you put something in your cart, NCL doesn't process anything until you put in the payment info. My first thought is that it's a glitch because of the payment plan.