Ok I tried this again. Deactivated the 2.8 Gold Cart plugin, activated the 188.8.131.52 plugin. Then went to Store>Upgrades and my API key was already entered/activated. To be sure I clicked “Submit” again, and it gave me a confirmation and told me my payment gateway list had been extended.
I navigate to Store>Settings>Payment Options but still don’t see Authorize.net listed as an available gateway.