Is anyone selling working-out-of-the-box / ready-made btc and btc lightning payment terminals for merchants? What would be the easiest and most user-friendly (both for merchants and their clients) way to transform a smartphone, tablet or laptop into one of these things? Been thinking a little about this myself but haven't put pen to paper, or finger to keyboard, on this yet myself.

Let's say I find a willing merchant who wants to add btc and btc lightning as payment options. How could I transform an android smartphone or tablet into a payment terminal? Particular apps? A web interface? (I'm talking about physical merchants like coffee shops)