Always Carry $20 and Some Coins
We live in a world where online payments are accepted everywhere, even for small amounts. However, paper money can always be used, even if an ATM malfunctions. Make sure you always have $20 and some coins in your wallet for situations where you can't pay with your debit card. Also, discipline yourself not to spend it when it's convenient. Save it strictly for when you have no other payment option.
