title: Best way to model a quota system for a subscription messaging service
body: <p>I have a rails application that has a subscription aspect with three plan levels depending on price tier. For example, 0-1000 messages is $10, 1001-10000 is $20, and a $0.01 surcharge on both for going over the quota amount.</p>
  
  <p>Each User has many Messages. What's the best way (high level) to keep track of each user's message usage and overages and charge them accordingly?</p>