Pompeii tickets

Why not purchase from the official site?
You don’t need to but you will likely pay more on Viator

I doubt they ever sell out

https://pompeiisites.org/en/

I had booked with Viator but read today that the official seller of
tickets has changed as of March.

As above, Viator has never been the 'official' ticket seller for this attraction or any other, for that matter. It doesn't mean that you don't have tickets but the link Christine has kindly provided is the official one with the correct pricing.

There is also the option of walk up ticket sales.

Were you getting something from Viator in addition to the tickets? We purchased our tickets when we arrived to the site.