You asked about OEM licnesing/support.
The Single Node is available to you under a BSD license; you can effectively do what you want with it provided you fulfill the copyright notice and disclaimer requirements https://github.com/EventStore/EventStore/blob/master/LICENSE
There are many organisations who will use the Event Store (single node) in similar ways to yourself, some for internal systems some in systems for sale. Quite a few have asked for support on the single node version, many of these are going to be running multiple instances on EPOS terminals, etc. just like you. It wouldn’t work for you if we charged for support for each of these, in your case 300 or so (although I would like that!).
Therefore, the OEM licensing/support is available on both the single node and HA version at exactly the same price. Although you don’t need it to distribute the single node version with your product, support might be a good idea to ensure you keep those tills ringing. If any of your store/customers have such a large store that they need the HA version then that is available to.