What $5 can do to you

Am at West Coast Plaza chilling at a cafe when i remembered that I have a $5 Gong Cha voucher to redeem. I entered Gong Cha. The lady with the cap and ponytail asked to take my order. I handover the $5 voucher and made my order. I did that as usually service staff expects you to give the voucher first before taking the order in case they need to key in or something. I asked the staff how much i needed to top up. She then replied loudly: "GIVE ME FIFTY CENTS". Her colleague laughed. She asked: Ni Zai Siao She Me? Then her colleague said" Yin Wei Ni Suo Give Me Fifty Cents!" She then grumbled something inaudible. I thought that it was her way of talking. After she was done with the cashier, she removed the receipt with a quick swipe and held the receipt in her hand and tilted her head and stared at me. I had to bend over the counter just to get the receipt from her. Then as i was waiting for my drink to be ready, her colleague said that was the 3rd coupon redeemed for the day, if not.. (she stopped upon seeing that I was looking at them). They then laughed amongst themselves. After I was home and resting, I realised that they were actually being very rude. What is $5 when you are just an employee paid by the hour with no commission to speak of? Do you see me as a cheapskate so you are acting that way? It's not as if you were the owner and you worry that you were making a loss right? I felt that I was bullied without even knowing.. so shortchanged! Next time I shall speak out if i realised that I was treated rudely/unfairly!