This sounds suspicously like a disguised Barry Hempel thread.
You would be able to calculate costs as the owner and provided that they are reasonable, then that would be ok. Keep in mind that for your average 4 seater you'd be talking over $100 per hour to be realistic so you would need to be charging the appropriate amount on paper. I have the same issue with my aircraft.
A notice board at the local supermarket would be commercial. If you were staying in the backpackers and met the foreigners as friends, then that would be ok.
An email to your workmates would not be.
A notice up at your employer would be a slightly grey area, but probably legal, especially if private behind closed doors.
It's all about what your lawyers tell in your defence, so when you get caught, lawyer up straight away.
The alternate solution would be to obtain a PPL and an AOC like everyone else that wants to be a flying ace / legend.