Hi Daniel,
After the first 30 days of an Azure Free account, you need to upgrade to Pay-As-You-Go. You do not need to upgrade your Azure support plan to the Developer plan (which is $29/month). You can select the Basic support plan instead, which is free, but only includes Billing/subscription support.
As long as you limit your use to the included Free services there will not be any charges each month.
Since you are not getting much traffic you could keep your static web app on the Free hosting plan, and assuming you are not using any other Azure services, there wouldn't be any charge each month.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP