Build the wallet payment

import { Buffer } from 'buffer';
import { PublicKey, Transaction, TransactionInstruction } from '@solana/web3.js';
import { createTransferCheckedInstruction, getAssociatedTokenAddressSync } from '@solana/spl-token';

function decimalToBaseUnits(value: string, decimals: number): bigint {
  const match = /^(\d+)(?:\.(\d+))?$/.exec(value);
  if (!match || !Number.isInteger(decimals) || decimals < 0 || decimals > 9) {
    throw new Error('Invalid token amount');
  }
  const fraction = match[2] ?? '';
  if (fraction.length > decimals && /[1-9]/.test(fraction.slice(decimals))) {
    throw new Error('Amount exceeds token precision');
  }
  return BigInt(match[1]) * 10n ** BigInt(decimals)
    + BigInt(fraction.slice(0, decimals).padEnd(decimals, '0') || '0');
}

// checkout is the data returned by your server after creating a checkout.
// Only continue while awaiting_payment, before payment_deadline,
// and if this buyer has not already submitted a payment for it.
const { treasury_ata, usdc_mint, decimals, memo } = checkout.payment;
const mint = new PublicKey(usdc_mint);
const amount = decimalToBaseUnits(String(checkout.total_usd), decimals);
const { blockhash } = await connection.getLatestBlockhash();

const transaction = new Transaction({
  recentBlockhash: blockhash,
  feePayer: buyerPublicKey,
}).add(
  createTransferCheckedInstruction(
    getAssociatedTokenAddressSync(mint, buyerPublicKey),
    mint,
    new PublicKey(treasury_ata),
    buyerPublicKey,
    amount,
    decimals,
  ),
  new TransactionInstruction({
    keys: [],
    programId: new PublicKey('MemoSq4gqABAXKb96qnH8TysNcWxMyWCqXgDLGmfcHr'),
    data: Buffer.from(memo, 'utf8'),
  }),
);

// Ask your connected wallet to sign and submit transaction.
// Send the signature to your server immediately after submission.
// Your server registers it with the checkout payment endpoint.