Or just use the great feature on the new EFTPOS machines in coles....
They allow you to do an EFTPOS transaction while the person is scanning your shopping for whatever amount you want, then when shopping is all scanned, it tells them how much change to give you. i.e. you put in a total of $100, shopping is $34.55 means they have to give you $65.45 in change....
Only problem is none of the staff know this feature exists and they either don't give you the change, which tends to result in an argument or just stand there looking stupid until I tell them how much money to give me. Hell, one even tried to give me the full $100 in cash, but given I shop there everyday and am well known by most of the staff, I felt kind of bad so I explained it to him.