Thanks Leonard and Santosh for your quick response.
Below are the configurations for the copying requirements in billing (tcode: VTFA). Should I need to change them in order to combine invoice collectively? Please advise.
Header
Copying requirement: 001 (Item/order-related)
Item
Copying requirement: 002 (Item/order-related)
Data VBRK/VBR: 001 (Inv.split (sample))
Eddy Au