Ethereum Public Chain Payment Gateway

I am building a service that must accept one-time customer payments directly on a public blockchain. My preferred rail is Ethereum, so the core of the job is a production-ready smart-contract–plus–API stack that lets me generate unique payment addresses, confirm the transfer, and surface the confirmation back to my app in real time.

Here’s what I need the finished package to do:
• Solidity smart contract deployed on Ethereum mainnet (initially Kovan/Goerli for testing) that logs each payment, enforces one-time use, and emits an event my backend can listen to.
• Lightweight Web3 interface or RESTful webhook that notifies my server once a transaction reaches a configurable confirmation depth.
• Simple front-end snippet (HTML/JS) that shows the client the exact amount in ETH and the destination address or QR code.
• Clear developer documentation, including how to redeploy the contract, rotate the receiving wallet, and test locally with Hardhat or Truffle.

Acceptance criteria
• A payment sent from MetaMask is reflected in my dashboard within the confirmation window we agree on.
• Contract passes unit tests covering failed sends, replay protection, and correct event payloads.
• No external dependencies beyond standard Ethereum tooling and open-source libraries.

If you have existing Solidity, Hardhat, or Web3.js experience and can demonstrate past payment flows you’ve built, we can move quickly.