At your own risk.  I ran out of 500g satchels and considered posting in a 3kg satchel (folding it & processing the postage label as  500g) but decided to contact Click & Send first.  Their answer: No.  They only allow use of appropriate satchel. Their inflexibility is frustrating because basically, if you're posting within the 500g weight, you should be able to parcel it any way you want.