I would like to install a payment gateway for a website i am making for a client in Australia. I know there are some gateways such as authorize.net packaged with Zen cart. If I am to use these gateways, what do I have to make sure first? Do I have to make sure the ISP supports these payment gateways?

If the ISP provides their own payment gateway, how likely is it that it is compatible with zencart?

I am quite unsure about how the payment gateways work. If you have experience about this, pls let me know.

Cheers!