What is FPX?
Financial Process Exchange, Malaysia's online-banking rail. The customer logs into their bank and approves the payment. Run by PayNet; almost every gateway supports it.
When a customer picks FPX at checkout, they are redirected to their own bank's login page, approve the payment there, and get sent back to your site. The money moves as a direct bank transfer, so there is no card involved and no chargeback mechanism like the card networks have.
For merchants, FPX is usually the cheapest rail: most gateways price it as a flat ringgit amount or a low percentage per transaction. Because the customer authorises the payment inside their own bank, failed-fraud risk is low, but a customer must have online banking set up to use it.