There is no capacity limit for each tier. Your service will not be interrupted by data usage fluctuation or tier change.
For example, if the total data you exchanged this month increases from 1 MB to 744,000 MB, the monthly pricing will automatically be adjusted to Tier 2. The price will be 743,800MB * 0.0007 = $521
You do not need to do anything, and your messages will be delivered and received without delays.