Stay on Free
Free is a plan you choose, not a state you fall into. Nothing happens to your account until you elect it.
Choosing Free at signup
On the payment step, alongside the card form, there is an option to continue on the free plan. Taking it records your choice and lets you into the app. No card is collected and no payment rail is contacted.
The step says plainly that there is nothing to pay and that you can upgrade later from “Billing”.
Choosing Free later
From the “Billing” page you can move to Free the same way you would move to any other plan.
If your account is not currently paying, that is all there is to it.
Moving from a paid plan to Free
If you are paying, electing Free means cancelling the paid subscription, so the app will not do it silently. It stops and asks you to confirm first, and the confirmation names the plan you are leaving and the date your current period ends. Only once you have confirmed does anything change.
This is the one case where choosing Free has a consequence worth reading before you click.
Shopify-billed accounts
If your plan is billed through Shopify, moving to Free is handled on the Shopify side, and Shopify ends a paid plan immediately rather than at the end of the period. The app states this where it applies.
What Free actually gives you
Free has real allowances and they are enforced by the server, not merely displayed. When you reach one, the action is refused rather than quietly dropped, and the app tells you which limit you met.
The figures are on the plan cards and on the usage panel. They are not repeated here. See Usage and limits.