Дмитрий, предложение "The parcel was delivered" употреблено в The Past Simple Tense. Это грамматическое время употребляется, когда мы говорим о каком-либо времени, которое совершилось в прошлом. Грамотно было бы добавить, когда совершилось действие. Например, The parcel was delivered yesterday/two days ago/on Thursday. Предложение "The parcel is delivered" употреблено в The Present Simple Tense. Это грамматическое время употребляется, когда говорят о регулярно совершающемся действии. Например, Parcels are delivered two/three times a month. Если это не регулярное, часто повторяющееся действие, а совершилось только что, то лучше употребить The Present Perfect Tense. The parcel has been delivered. - Посылку принесли. (Имеется ввиду, что ее принесли только что, недавно)

Imagine that a friend tells you, my car broke down, can you please deliver this parcel to my mom? You take the parcel, deliver it to the mom, and as soon as you exit her house, you send your friend a text saying, "parcel is delivered." This conveys the message to your friend that you just did it. The action happened probably a minute ago. If you send the message, "parcel was delivered" then your friend could reply with, "how long ago did you deliver it?" By using was, you convey the idea that it happened in the past, some time ago. Also, in the USA, parcel tends to be more formal; usually people say, package. If you buy something online, you would say, when is my package arriving...

When we say "Parcel is delivered", we are drawing the reader to the idea that the action was completed recently and the effect of the action is still present. When we say "Parcel was delivered", we are trying to focus on the idea that the action completed further in the past.

The difference is that, first one determines the delivery in past and later one determines the delivery of parcel in present