[yummyfajitas]

For the use case described here scalability is irrelevant. The OP is processing hundreds of real world shipment updates per day. I'm pretty sure that Postgres can handle that on a $10-20 digital ocean droplet.

A back of the envelope calculation. Assume each shipment brings in $0.10 in profit to his company. Suppose at some point he needs to scale up to 100,000 shipments/day. His revenue is now $10,000/day.

A "Performance One" Rackspace server costs $1250/month and Rackspace isn't known for being cheap. A Hetzner box with all the things added (12TB HD, 2.4TB SSD HD, 384GB RAM) costs 831 Euro/Month.

[pkolaczk]

Scalability is irrelevant, but availability/reliability is. I simply don't get why you insist on comparing MongoDB to RDBMS while there are better NoSQL alternatives, which would be the right tool for the job.