do you only pay for international roaming if you use the phone or do you get charged $10 per day even if you don't use it?
You have to nominate the days you want a travel pack. You will be charged regardless of use.
Note if you buy consecutive days then the data allowances of each day can be used anytime in the range (i.e. 3 days of packs gets you x data which can all be used on day 1, day 3 or over all days.
Note current Optus post paid plans include genrous travel allowances for Zone 1 countries. You might have already or be interested?