Implement Game Top-Up System

I’ve built a small web-based game and now need a seamless top-up feature so players can purchase in-game currency and power-ups directly from the site. I’m planning to integrate a standard payment gateway—whichever solution you recommend that reliably handles cards, PayPal, or similar options is fine—so long as checkout feels native and secure.

Here’s what I’m after:
• Add a “Top Up” flow that pops up from the current lobby, lets users choose fixed currency bundles or individual power-ups, and then routes them through the gateway.
• No bulk-buy discounts or dynamic pricing; every item keeps a single, consistent price.
• After payment, credit the player’s account immediately and show the updated balance without page refresh (AJAX/WebSocket is okay).
• Provide a lightweight admin panel (or config file) where I can add, edit, or deactivate currency packages and power-ups without touching code.

The site is written in PHP with a MySQL backend; JavaScript handles the front end. If you have a different stack in mind for the payment widget, I’m open to it as long as it drops cleanly into the existing codebase.

Please outline your approach, the gateway you’d recommend, and any similar projects you’ve completed. Once we agree on the plan I’ll give you repo access and test keys so we can roll this out quickly.