Are you sure they cheat you, everything was done in front of you, you can ask why to them. I always did.

I presume that you sending to Zone 2 area (Within Peninsular)
Zone 2 - 1st 500gm RM4.50, Add 250gm RM1.00
so 4.50 + 1.00 + 1.00 = RM6.50 + tax.

but wait.... did you use big box....?
'Rates are based on the total actual weight or the total dimensional weight of all packages in a shipment, whichever is greater'
even if the item lighter but use big box, the rates is higher.

I like Poslaju, yesterday they can't find my house number which is 18 and found number 16. They call me to ask... my answer 'just next to it'. Most of time Poslaju will call me...

but Nationwide will not. that happen to me a few days before raya... had to rush to pick up at their place in the middle of jam (45 minutes before bukak puasa) 

forgot to mention

+ Handling charge + fuel surcharge (10% + 15%)
+ tax 6%

6.50 + 25% = 8.125 + 6% = 8.6125

look like you getting discount....