This must be pieces of mail per year? To me "parcel" means more like a package.
Some web searching confirms this and also says a little over half of that is marketing mail. 60B pieces of spam per year. And I don't know about you, but excluding marketing mail, I'd say about 90% of the remainder still goes directly in the bin for me. And even though I open probably less than 5% of what is sent to me, maybe less than 1%, STILL much of that could have been an email.
I don't doubt the USPS provides a whole lot of value and I don't want them to go down to 1 day per week or anything. But that 100B number looks like it includes a whole lot of negative, zero, or low value items.