Crypto Payment Acceptance for Merchants in Mobile App

Implementing Crypto Payment Acceptance via Mobile Application (Merchant)

Merchants need to accept crypto payments at point of sale or online shop. Mobile app is a cash terminal: enter amount, show QR to customer, await confirmation, record sale. All must work fast and without complex setup.

Generating Payment Address and QR

Generate new address per transaction — simplifies matching "which payment for which order". HD wallet (BIP-32/BIP-44) allows address derivation: m/44'/60'/0'/0/N.

// iOS — deriving next address from HD wallet (secp256k1)
import HDWalletKit

func nextReceivingAddress(masterKey: HDPrivateKey, accountIndex: UInt32) -> String {
    let derived = masterKey
        .derived(at: 44, hardened: true)
        .derived(at: 60, hardened: true)   // ETH
        .derived(at: 0, hardened: true)     // account
        .derived(at: 0)                     // external chain
        .derived(at: accountIndex)
    return derived.publicKey.ethereumAddress
}

Store accountIndex locally, increment per payment. Server side should monitor all used addresses.

QR Code with Exchange Rate Timer

Crypto rates change. To ensure customer pays exactly the right amount:

1. Merchant enters fiat amount (1,000 RUB)
2. App requests current rate (CoinGecko /simple/price?ids=tether&vs_currencies=rub)
3. Calculates crypto amount with buffer: amount_usdt = fiat_amount / rate * 1.005 (0.5% slippage)
4. Locks rate for 15 minutes, shows timer
5. Generates QR with URI: ethereum:0xUsdtAddress@1/transfer?address=0xMerchant&uint256=<raw_amount>

// Android — calculating crypto amount with buffer
fun calculateCryptoAmount(
    fiatAmount: BigDecimal,
    ratePerUnit: BigDecimal,
    slippagePercent: BigDecimal = BigDecimal("0.5"),
    decimals: Int = 6  // USDT: 6
): BigInteger {
    val slippageFactor = BigDecimal.ONE + slippagePercent / BigDecimal(100)
    val cryptoAmount = fiatAmount.divide(ratePerUnit, 10, RoundingMode.HALF_UP) * slippageFactor
    return cryptoAmount.movePointRight(decimals).toBigInteger()
}

On timer expiry — update QR with new rate. Warning: "QR rate expired, updating price".

Awaiting and Confirming Transaction

After showing QR — app listens for incoming transactions to payment address. Two options:

Polling via API: Query Etherscan API / Blockscout every 5–10 seconds: GET https://api.etherscan.io/api?module=account&action=tokentx&address={paymentAddress}&startblock={latest}

WebSocket from server: Backend monitors blockchain and sends event via WebSocket or push on received transaction.

First option simpler but costly at scale. Second is correct for production.

Payment Statuses

// iOS — payment status state machine
enum PaymentStatus {
    case awaitingPayment(expiresAt: Date)
    case detected(txHash: String, confirmations: Int)
    case confirmed(txHash: String, amountReceived: BigDecimal)
    case underpaid(expected: BigDecimal, received: BigDecimal)
    case expired
    case failed(reason: String)
}

On detected — green screen "Payment detected, awaiting confirmations (1/3)". On confirmed — cash register ding, full success screen with amount.

Partial Payment and Timeout

If customer paid less — underpaid. Three strategy options:

1. Wait for additional payment until extended timer (1–24 hours)
2. Accept partially with debt record
3. Auto-refund (complex — requires withdrawal key storage)

Choose strategy in merchant app settings.

Transaction History and Reports

List accepted payments: date, amount in crypto and fiat at payment rate, status, TxHash (Explorer link). Filters by date and status. CSV export — for accounting.

Timeline: 5 working days: HD address per order, QR generation with URI format and rate timer, polling/WS incoming transactions, confirmation screen with sound, history with filters.