I’ve been down the path (before I bought my Zero Water jug) of buying RO water from a local shop. And previously from a chain of RO dispensing machines. Turned out that they didn’t change their water filters frequently enough and what I was really buying was 25 TDS water. And I blissfully ignorantly added TWW packets to that. Until I had the combination tested at a lab. It tasted fine/even good. But the chemistry was an espresso machine killer.
Now I use Zero water for espresso and BWT filtered water for everything else including drinking water. I add a sodium bicarb concentration to the Zero water, as per the water recipe used by many on this Forum.
I don’t go through a lot of Zero water filters, as I am the only espresso consumer in the household. We use more BWT filters, but not enough to warrant installing a filtration system.
I’d use a TDS meter and an aquarium water testing kit on any water you buy. Caveat emptor.