[karaterobot]

It seems like you're anxious because you assume that asking for a raise is like burning a bridge with your employer. They don't see it that way: they'll just say "we can't justify increasing your salary at this time", or more likely they'll offer you a small raise (less than you want), since it costs them so little to do so.

They'll never say "how dare you even ask that! get out of here!". They'll never say "hmmm, who should we lay off? probably that ungrateful so-and-so who had the temerity to ask for more money". They'll just say yes, or no, and move on.

(Negotiating the amount of the raise is an "advanced" problem. I've never done it myself, though I've heard tales of legendary figures who have accomplished this epic feat)

Just be polite, state your case, but do not make an ultimatum like "I have another job lined up if you say no," because that tells them you're looking to leave, and they might just give you that raise just long enough to find a replacement while you finish up the work you're doing right now.

I would make the case to your manager, phrasing it in terms of how you've made exceptional contributions that earned the business more money, and that in so doing you took on responsibilities above your stated job description.