As this include shopping trip, what are the guidelines that I need to know more?

The Company is strictly following the rules and regulations implemented by both Malaysia and Singapore Immigration and Customs. It is not the responsibility of the Company if a passenger is detained by the immigration officer due to Immigration laws from both countries.

You are advised to proceed to the Customs’ RED CHANNEL for declaration if you have any dutiable or taxable goods exceeding your duty-free concession or Goods and Services Tax (GST) relief for Singapore, controlled or restricted goods; or prohibited goods.

For more information, on the custom regulations, please refer to:
Singapore Customs: https://www.customs.gov.sg/
Malaysia Customs: http://www.customs.gov.my/en/tp/