I assume by backglass, you mean the scoreboard section of the pinball machine? you should be fine.
still, you definitely left some $$$ on the table, i'd say. hope they providing beer & pizza at least
Even as a cash job, I'd be printing & supplying at that price. you'll have at least 4hours of labor into this by the time you're done... not to mention materials.
buuuuuut at this point, just do it. should give you a few years before having an issue, as it will be indoors.
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also, first-surface is face application. second-surface is applied to backside of substrate with intention to be viewed through the glass.
second surface is usually trickier, as imperfections in your application stand-out that much more.