Hi,
In the screenshot you posted it is asking you to choose a support plan. Basic is no extra charge, so choose that option for now. Under Basic support you can submit cases for Subscription or Billing issues only.
If you need help with a technical issue you can easily sign up for one of the paid options later.
No, Pay-As-You-Go does not ensure that your website remains operational even during periods of high traffic. To do that you would need to configure scaling and test. For example, if your website is hosted on App Service, you would configure the size (vCPUs/RAM) of each instance your app runs on, minimum number of instances, under what conditions should it scale out to more instances, etc.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP