"She gave me a bag of fruit."
There is no strict definition. Usually 一袋 is an open bag and 一包 is a closed bag, but it may vary.
Duolingo is trying to build an equivalence between 了 and English past tense, which is not true. I have no strong objection except that they should make a remark it's a simplified model for easier learning.
Both with 了 and without 了 can be a good sentence. Just an isolated sentence like this is insufficient to tell it is needed or not. Basically, just a description of the action does not require 了; to indicate a change of state of facts or the resultant state of facts due to the action, we need 了.