I run an online platform where clients create and manage customer-engagement SMS campaigns. I now need PayPal Checkout woven seamlessly into the site so users can pay for their messaging credits without ever leaving the flow of campaign creation. Here is what I’m after: • Add the latest PayPal Smart Buttons to the purchase page that sells SMS credit bundles. • Support both Sandbox and Live modes so I can test before going public. • Once a transaction is authorised, instantly credit the buyer’s account and trigger our existing confirmation email/SMS. • Capture and store PayPal transaction IDs in my MySQL database; I’ll provide the table schema. • Make sure the integration works smoothly on desktop and mobile, and that errors (cancellations, declines, time-outs) are handled gracefully. Tech stack you’ll touch: PHP 8, JavaScript (vanilla), and a REST-style backend API. If you prefer using the official PayPal PHP SDK or JavaScript SDK, great—just keep the code clean and well-commented. Acceptance criteria A purchase in Sandbox mode: 1. Shows PayPal pop-up, completes payment, and redirects to the success page. 2. Credits appear in the user account instantly. 3. Transaction record is written to the database with status “COMPLETED.” Once that works, we’ll flip the switch to Live mode. I’ll supply server access, API keys, and sample user accounts as soon as we start. Looking forward to a robust, tidy integration that lets my users pay with confidence.