explain to me:

Bangkok		BKK		TH		15
		BKK		TH		
Brisbane		BNE		AU		50KG
		BNE		AU		
Johannesburg		JNB		ZA	6000	10
					-6000	10
					-3000	10
					-2000	10
what is "-50" in row 3? Can a piece of freight have negative weight?

The blank rows in your data column will be a problem. You need unique codes in column A or in column C.

Then fix row 3 to be meaningful steps of weight categories and just adapt the formula I posted earlier to reflect the rows and columns you are using right now.

cheers.