Personally, I avoid paying with credit card online as much as possible. Why? Because your data can be lost or robbed by hackers. Also if you have any problem with any online dealer you would have no support.

I have a Paypal account, which uses my credit card data without displaying it to anybody. If anything goes wrong with the retailer you have the support of Paypal, and it is also easier and faster to get refunds. That is my experience.

Every retailer is different. It is up to them if they accept credit cards or paypal or both. So you cannot do by follow their rules. It is the same when you go to a real shop. They accept Visa but perhaps not Amex, you cannot force any retailer to use Amex, right? The same serves for online retailers.

And what exactly does mean you want to pay with OZ? in Australian Dollars?