Was @ Kopitiam, Bukit Panjang Plaza two days back and they only accept their Kopitiam Prepaid Card for payment and do not accept cash at all.
The min purchase of each card is $5. And this apply to all outlets.
I witnessed a few unhappy customers when I was there. There are some who dropby to get a cup of drink or a fruit and yet they would need to purchase the card before they could do so.
There are also students who only have $3 to have their meal and now they simply can't as their parents are not with them and they do not have enough cash to purchase the card.
As for me, only after I ordered my food then I was told that they do not accept cash.
