My assumption is that they can't, and the plan is to burn through VC money with the hopes of getting some sort of eventual deal with the postal service.
Assume they pay their pick-up guy $20/hour. Each customer is paying $5 for 12 pickups = 60 cents a visit. To break even they will need to do pick-up 33 mail boxes/hr. This seems like a lot in a suburb but it's possible in a dense city. They will probably need to tweak their model but it's not unreasonable.
It's totally unreasonable. The only place it makes even a hint of sense to do this is at the Post Office, before it goes to the actual customer. The customer could then choose scan, shred or deliver.
Maybe I'm just not clever enough to get it. But this just seems ludicrous to me.