The big problem with a cluster box is FULLNESS. That’s readily solved with what’s called a PARCEL LOCKER (in common parlance). Let’s say there’s too much mail for your regular box they stick it in one of the several parcel lockers in the cluster unit and deliver you the key in your regular box.
We don’t have cluster boxes or room for them. In our case, you see the postmistress and she keeps your extra mail stuck in a cardboard box. We have a waiting list just to get a post office box. The next town over just had their post office shut. This is just beginning.