There is really no need to be too concerned about the 3kg limit, if you go over that the next bag size is 5kg and only cost $14.50 vs $11.40 for the 3kg one. Both these bags now also have tracking included in the price, make sure you get the little blue tag off the bag and get the PO to scan and give you a tracking receipt.
The 5kg bag is also a lot larger
3kg = 405x310mm = $11.40
5kg = 510x435mm = $14.50
For my money it is $3.10 well spent.