It's accepting it now, 2 years later, which I think is making the difference between passato prossimo and imperfetto even more confusing. In this example, "I opened the envelope" appears to be a one time action in the past thus I would then translate it back in passato prossimo... "Ho aperto la busta" ... not what it started out with "Io aprivo la busta"
Flying_Blue: I agree with you. To follow up though I'd add that the imperfect would be justified if the context were: I was opening the envelope,...when e.g. something else happened. If on the other hand as you say, it's just a statement about a 1 time action, then the passato prossimo would be more appropriate.
I opened the envelope was accepted, it should be, in English we would not always use was doing or used to do in all the situations that need imperfect in Italian. This is one of my problems is speaking Italian as I use the perfect tense as often as I do in English, when often the imperfect would be better Italian
I think it would help if Duolingo would add the rest of the situation. If you open one envelope implied here because they used "the" you should not be able to use I opened the envelope. That would imply a one time action. I'm not sure why they are accepting it as valid. If you would say, I opened the envelopes every day at noon, then yes, but not the envelope. If it was a specific envelope that you received everyday, with let's say your daily company report, then again, it would make sense.