Payment processors with third party

Hi, I have a e-commerce web portal where I allow my clients to collect payments from their clients. I currently use a payment gateway that charges me a fee. I am a developer and have looked into building APIs with Braintree or stripe but can’t find a model where my clients get the money directly that their clients pay via the portal but I get charged the fee from the processor so I can then add my mark up and charge them. Both stripe and Braintree seem to want to charge the fees directly to my clients. I basically want a model where party A gets the money and party B (me) gets the fees. Has anyone tried it and has any idea how to go about it?