USD Gateway & Mass Payments Setup

My PHP site (https://salenumberone.com/) is fully built and live; the only piece still missing is a clean, reliable payment flow in US dollars. At the moment the checkout runs through PayPal only, and the percentages for each transaction type are already coded on the back end. What I now need is:

• Convert the existing gateway logic so every purchase is processed strictly in USD, with correct rounding and fee calculations.
• Enable mass-payment functionality that lets me trigger bulk payouts through the same gateway rules. The code should be written in a way that I can later plug in card processors or local-bank rails without rewriting everything.
• Stress-test the flow (sandbox and live) and document any configuration steps so I can replicate them on staging or future servers.

The project is entirely PHP/MySQL with no framework overlays, so native PHP, REST APIs, cURL, Webhooks and solid knowledge of PayPal’s Payouts endpoint will feel familiar. If you’ve integrated multi-currency logic before, please highlight that; I may add additional currencies later.

Delivery is the updated, commented source plus a short read-me that shows how to toggle between live and sandbox keys and how to initiate a mass payout. Once it passes a live transaction test in USD, the job is done.