why can't he accept cash? If that's what you want to give him I say go for it.

I think wine is lovely. But what about something like movie vouchers in case he doesn't drink?
Are Australian Post employees, Public Servants? If so, as Public Servants we can not accept gifts. Its described in our ethical standards.

If gifts are accepted they have to be declared. I think there is legislation/policies/procedures noted somewhere noting what steps have to be taken.

Maybe some chocolates and a lovely card??