PayZee

PayZee is a non-custodial payment infrastructure designed to solve the "last mile" problem of cryptocurrency: universal spendability. While crypto assets have matured, the bridge to real-world commerce remains broken, usually requiring slow, high-fee off-ramps to centralized bank accounts. PayZee eliminates this friction by allowing users to spend Sui and stablecoins at any online merchant that accepts credit cards—instantly.

#The Workflow The system utilizes a sophisticated "Bridge-Tab" architecture to bypass restrictive browser security (CSP).

1. Detection: A browser extension monitors e-commerce sites to identify checkout totals and credit card input fields.

2. Escrow: When a user initiates a payment, they are directed to a secure Sui DApp. Using Programmable Transaction Blocks (PTBs), the user locks the required amount (plus a 5% safety buffer) into a smart contract.

3. Issuance: Our FastAPI backend monitors the Sui ledger. Once the transaction is confirmed, it calls the Lithic API to issue a single-use, merchant-locked virtual Mastercard or Visa.

4. Settlement & Refund: The extension provides the card details for the user to complete the purchase. Once the merchant settles the transaction, any excess funds from the safety buffer are automatically refunded to the user’s Sui wallet via an asynchronous worker.

#Why Sui? We chose Sui for its sub-second finality and PTBs. In a checkout environment, a user cannot wait minutes for a block to close. Sui allows us to verify the escrow and issue the fiat card in real-time, matching the speed of traditional banking while maintaining decentralized custody.

#Security & Innovation

1. Single-Use Cards: Every card generated is programmatically destroyed after one transaction.
2. Merchant Locking: Each virtual card is cryptographically bound to the specific merchant's ID, preventing fraud if the card details are intercepted.
3. The Slippage Buffer: Our proprietary logic handles the dynamic nature of e-commerce (taxes/shipping) by over-collateralizing the initial lock and handling micro-refunds on-chain.

#Key Technical Highlights

1. Backend: Python/FastAPI with Lithic SDK integration.

2. Blockchain: Sui Move smart contracts for secure asset escrow.

3. Frontend: React/Vite for the DApp and a Manifest V3 Browser Extension.

4. Real-time: Utilizing Sui's event-driven architecture for instant backend triggers.

PayZee is built on a high-performance stack designed for financial precision and sub-second execution. The architecture is split into three core layers: the Sui Move smart contracts, a FastAPI orchestration backend, and a React-based browser extension/DApp suite.

#The Core Logic (Sui & Move) We leveraged Sui’s Programmable Transaction Blocks (PTBs) to create a streamlined escrow flow. When a user initiates a payment, our Move contract locks the USDC/SUI tokens. We utilized Sui’s event-driven architecture to emit real-time signals that our backend picks up instantly. Sui was essential here because traditional block times (10-15s) would cause e-commerce checkouts to time out; Sui’s sub-second finality makes the bridge feel "native" to the web.

#The Fiat Bridge (Lithic Partner Tech) We integrated the Lithic API to handle the legacy banking side. Once our backend verifies the Sui transaction, it programmatically generates a Single-Use Virtual Card. Lithic’s "Merchant Locking" feature was a game-changer for us—it allows us to restrict the generated card to only work at the specific merchant the user is currently visiting (e.g., only Amazon), providing a layer of security that traditional credit cards lack.

#The Orchestrator (FastAPI & Python) The backend serves as the secure "Handshake" between the public ledger and the private banking rails. We used Python’s FastAPI for its asynchronous capabilities, allowing us to manage multiple concurrent checkouts. This layer handles the currency conversion logic and manages the sensitive communication with the browser extension via encrypted WebSockets.

#The "Hacky" & Notable Details To make the demo work seamlessly on high-security sites, we implemented a "Bridge-Tab" workflow. Browser Content Security Policies (CSP) often block wallet popups on sites like Amazon. We solved this by having the extension open a "checkout tab" on our own domain, allowing a clean wallet signature environment.

Additionally, we built a custom Slippage & Refund Worker. Because e-commerce totals often change slightly due to dynamic tax and shipping, we "hackily" but effectively implemented a 5% over-collateralization buffer. Our backend acts as a micro-accountant: it waits for the Lithic transaction.settled webhook, calculates the exact change down to the penny, and automatically triggers a Sui refund transaction back to the user. This ensures the user never overpays, even if the merchant's final price shifts.