So what's the best coin to buy a cup of Coffee with? Transaction cost must not be expensive and the transaction must complete before the Coffee gets cold... 
maybe you could consider using Dogecoin or TRX, because both of these cryptos have transaction fees that are cheap, fast or even as fast as before your coffee cools. btw Post OP about 2 years ago or to be precise in 2018, how is the latest development of your coffee shop? is transaction service using crypto legal in your country?