Clarification on Azure Firewall multi-AZ pricing

I'm looking for clarification on the pricing for Azure Firewall as I seem to be finding conflicting information.

As I understand it, when deploying Azure Firewall I can choose to have it within a single AZ or across multiple AZ's. There is a 99.95% SLA in a single AZ or 99.99% in more than one AZ.

So, in order to get 99.99%, I effectively need to deploy/pay for a pair (two) units?

I normally check out some blogs on Azure resources and I spotted this post (and I've seen others with similar information)

https://www.reblaze.com/blog/exploring-azure-firewall/

It mentions

There is no additional charge for running firewalls in an availability area. However, inbound/outbound data transfers to or from an availability zone incur additional costs.

It would seem that I don't get charged for the additional unit?

Also, I've had a look for Az data transfer costs

https://azure.microsoft.com/en-gb/pricing/details/bandwidth/

Which states it is free.

​

Maybe I should just be ignoring this and similar blogs? Hoping that someone might be able to shed some light!

​

Thanks