Data Pool Billing

I’m trying to understand the billing of the flexible usa plan vs a data pools.
From the dashboard and the hologram.io webpage it says I’m billed at $.08/MB using the flexible USA plan, plus the additional $.70/device/month. Now the Data Pool, which is marketed as being a cheaper option starts with 100MB for $30. Is there something I’m missing about the billing? Just trying to avoid a large unexpected bill.
Thanks!