I am considering using the service but have some questions:
I stay on the 1 GB plan that I share with my wife (and we almost never exceed that limit). In addition to the stated $12, I end up paying $4.87 in unavoidable taxes (911, communication service, Universal Connectivity Charge) but no additional charges for access or anythin related to the number of lines in use. I don't know whether the taxes are a percentage or flat fees, but they're probably the same as AT&T. In other words, no extra charges other than the tax man. Oh, and I paid 6 cents for a call to Canada.
I don't know about any workarounds for your Surface. The closest I've ever tried is using my Galaxy S9 connected via DeX to an HDMI input of a monitor (i.e., no additional device involved), but I don' tthink that's what you're after.