How I Built a Crypto Payment System with web3.py — No Stripe, No Fees

#python #web3 #crypto #blockchain

I got tired of payment processors rejecting accounts and taking a 3% cut. So I built my own payment system using web3.py and USDC on Polygon.

How It Works

The core logic is surprisingly simple. USDC is an ERC-20 token, and every transfer emits a Transfer event. web3.py lets you read these events directly from the blockchain — no API key required.

from web3 import Web3

w3 = Web3(Web3.HTTPProvider("https://polygon-rpc.com"))
USDC = "0x3c499c542cEF5E3811e1192ce70d8cC03d5c3359"

def verify_payment(tx_hash, expected_amount=9.99):
    receipt = w3.eth.get_transaction_receipt(tx_hash)
    for log in receipt["logs"]:
        # Check Transfer event signature
        if log["topics"][0].hex() == TRANSFER_TOPIC:
            # Decode recipient and amount
            recipient = "0x" + log["topics"][2].hex()[-40:]
            amount = int(log["data"].hex(), 16) / 1e6  # USDC has 6 decimals
            if recipient.lower() == MY_WALLET.lower() and amount >= expected_amount:
                return True, amount
    return False, 0

That's it. No Stripe SDK, no API keys, no webhooks. Just direct blockchain interaction.

Why Polygon?

Gas fees: ~$0.01 per transaction
Confirmation: 2-5 seconds
USDC is stable (pegged to USD)
Native web3.py support (EVM compatible)

The Full Stack

Flask for the web app
web3.py for on-chain verification
SQLite for tracking payments
Cloudflare Tunnel for public access

The entire payment integration is about 100 lines of Python. No third-party payment dependency.

Trade-offs

The obvious downside: users need a crypto wallet and some POL for gas. But for a developer tool audience, this is increasingly common. And the upside is massive: