After you've obtained your Google Maps API Key as explained in the How to add a Google Maps API Key to your StoreRocket store locator article, the next step is to setup alerts for your Google Maps API usage in order to avoid unexpected charges and be notified when you reach a specific threshold.
Go to your Google Cloud Platform Console, click the menu icon at the top left corner and click Billing.
Click on Budgets & alerts and then click on Create budget.
In the page you will see the form to create a new budget.
Give a name to your budget
Select the proper billing account
$200at the budget amount
The default budget alerts will be pre-filled, feel free to change them if you want
When you are done, click Save.
Google will now send you alerts if and when you will reach those thresholds. In any case remember that Google won't be billing you without notifying first.