As mentioned on our App Page, the Free tier is only applicable to the FIRST 100 variant.
Merchants are notified about this again before installing the app:
We also have a variant counter to show merchants how many variant had been selected at the top of the App page:
Once the bar is filled up, the paid tier will start billing.
Therefore, if you are testing the app out, we recommend that you activate just a small portion of your products instead activating the whole store.
Should you have any inquiries about our billing, please do not hesitate to contact us at firstname.lastname@example.org