Domestic payments and collections denominated in SGD via FAST or PayNow are free of charge, both for topping up your OPAL account and for making payments from it.
OPAL is charging competitive fees. Using OPAL, there are no hidden fees.
When using the OPAL platform to make an outbound transaction, the platform will provide you with the total cost (if any) of the transfer before you approve it. The final amount that is presented on the platform is what you will pay.