Changi Airport Group review: attitude issue

I've purchased TP TEA for so many times, I always feels that the service of tp tea is very well, until today (13th Nov around 10:30pm), the cashier girl was very nice as always, but when I collect my 2 drinks, I ask for plastic bag from a female with long hair and also wearing big glasses in a nice attitude, the staff somehow give me a very rude attitude like I did something wrong to her, she even slam the bag on the table and let me pack the drinks by myself, I don't mind to pack myself but the attitude makes me feel irrespective, I know everybody would feel tired after working a long shift, but if you working in service industry you should know how to hide your emotions, especially I never give her any bad attitudes, I hope she can learn how to serve a customer properly.