Issue tickets against a settled USDC payment

import { createTransferCheckedInstruction, getAssociatedTokenAddressSync } from "@solana/spl-token";import { PublicKey, Transaction, TransactionInstruction, sendAndConfirmTransaction } from "@solana/web3.js";const MEMO_PROGRAM_ID = new PublicKey("MemoSq4gqABAXKb96qnH8TysNcWxMyWCqXgDLGmfcHr");// Public payment details fetched by your server. Never expose your partner API key.const { treasury_ata, usdc_mint, decimals, memo_prefix } = detail.data.payment;// 2. Exact amount, fixed-point. Floats will drift you off by base units.function decimalToBaseUnits(value, decimals = 6) {  const match = /^(\d+)(?:\.(\d+))?$/.exec(value);  if (!match || !Number.isInteger(decimals) || decimals < 0 || decimals > 9) {    throw new Error("Invalid token amount");  }  const fraction = match[2] ?? "";  if (fraction.length > decimals && /[1-9]/.test(fraction.slice(decimals))) {    throw new Error("Amount exceeds token precision");  }  return BigInt(match[1]) * 10n ** BigInt(decimals)    + BigInt(fraction.slice(0, decimals).padEnd(decimals, "0") || "0");}const baseUnits = decimalToBaseUnits(detail.data.ticket_price_usd, decimals) * BigInt(qty);// 3. transferChecked (NOT transfer) + exactly one memo.const mint = new PublicKey(usdc_mint);const tx = new Transaction().add(  createTransferCheckedInstruction(    getAssociatedTokenAddressSync(mint, payer.publicKey),    mint,    new PublicKey(treasury_ata),    payer.publicKey,    baseUnits,    decimals,  ),  new TransactionInstruction({    keys: [],    programId: MEMO_PROGRAM_ID,    data: Buffer.from(`${memo_prefix}:${externalUserRef}`, "utf8"),  }),);// 4. Wait for FINALIZED before submitting to Verity.const signature = await sendAndConfirmTransaction(connection, tx, [payer], {  commitment: "finalized",});