HI Ruud
You can certainly build for both scenarios - agent forwarded, and IVR forwarded - in a single Architect secure flow in Genesys Cloud. The question then just comes down to who your actual payment processor is and having Data Actions within the secure flow to handle the transaction, including any subsequent Data Actions that may be needed to post the transaction outcome into a CRM etc.
You may also want some logic in that flow to identify if the call came in via the agent transferring them via script or from a self-service IVR menu option. That way you can decide if you want to use the return to agent option or not and how you do error handling.
One question though, when you say "agent assisted", I'm assuming you mean agent forwarding them into a secure payment IVR as opposed to the agent actually being involved in entering credit card details for the customer. You could certainly have the agent pass in the amount to pay but you'll want to make sure you include in your secure flow some type of confirmation option for the customer to accept that's the amount to pay.
Just make sure when you do build the secure flow that you're aware of all your PCI etc compliance requirements and obligations.
Bottom line is if an organisation already has a payment provider/card processor, and they have REST services available, you shouldn't need to use any other third party type integration or solution.
------------------------------
Vaun McCarthy
------------------------------