Under $50" reminds me of my buddy's failed lawn mowing flyers.

Ever think about just taping a twenty to the back of your phone case? My buddy learned that lesson after a situation like yours. He was trying to pay a bridge toll and his phone just shut off, no warning. He had to dig through his glove box for loose change while people honked. After that, he started keeping a folded bill behind his phone, which is basically what @paige562 does. It seems dumb until you're the one holding up a line.