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Getting Fee Quotes from the Token Paymaster with SDK ⚡️

Users need to know the fee amount before they can send a transaction. In this tutorial, we will guide you through the process of getting a fee quote from the Token Paymaster for USDC. We will also show how to enforce a custom approval amount equal to the fee amount for a transaction, instead of asking the user for a max approval amount.

Set up an owner account

First, we need to set up an Owner for the Smart Account which will be used to sign User Operations (transactions) for the Smart Account.

import { privateKeyToAccount, generatePrivateKey } from "viem/accounts";
 
const privateKey = generatePrivateKey();
const account = privateKeyToAccount(`${privateKey}`);

Set up Nexus client with paymaster

Login to the Dashboard and setup a v2 paymaster. Let's configure a client for the Smart Account with a paymasterUrl to enable it. A bundlerUrl is required to submit transactions to the Network, which will initialize the Smart Account.

import { createSmartAccountClient, createBicoPaymasterClient, toBiconomyTokenPaymasterContext } from "@biconomy/sdk";
import { baseSepolia } from "viem/chains"; 
import { http, parseEther } from "viem";
import { privateKeyToAccount, generatePrivateKey } from "viem/accounts";
 
const privateKey = generatePrivateKey();
const account = privateKeyToAccount(`${privateKey}`);
 
const bundlerUrl = "https://bundler.biconomy.io/api/v3/84532/nJPK7B3ru.dd7f7861-190d-41bd-af80-6877f74b8f44";
const paymasterUrl = "https://paymaster.biconomy.io/api/v2/84532/F7wyL1clz.75a64804-3e97-41fa-ba1e-33e98c2cc703"; 
const baseSepoliaUSDC = "0x036cbd53842c5426634e7929541ec2318f3dcf7e"; 
 
const paymasterContext = toBiconomyTokenPaymasterContext({
    feeTokenAddress: baseSepoliaUSDC
})
 
const paymaster = createBicoPaymasterClient({
    transport: http(paymasterUrl)
})
 
const nexusClient = await createSmartAccountClient({
    signer: account,
    chain: baseSepolia,
    transport: http(),
    bundlerTransport: http(bundlerUrl),
    paymaster,
    paymasterContext
});

Calculating the USDC fee amount and enforcing a custom approval amount

Next, we will prepare the user operation that we want to send using prepareUserOperation and get a fee quote for USDC from the Token Paymaster using getTokenPaymasterQuotes. We will use the maxGasFee and decimal from the quote to calculate the fee amount in USDC and parse it using parseUnits.

 
import { createSmartAccountClient, createBicoPaymasterClient, toBiconomyTokenPaymasterContext } from "@biconomy/sdk";
import { baseSepolia } from "viem/chains"; 
import { http, parseEther, parseUnits } from "viem";
import { privateKeyToAccount, generatePrivateKey } from "viem/accounts";
 
const privateKey = generatePrivateKey();
const account = privateKeyToAccount(`${privateKey}`);
 
const bundlerUrl = "https://bundler.biconomy.io/api/v3/84532/nJPK7B3ru.dd7f7861-190d-41bd-af80-6877f74b8f44";
const paymasterUrl = "https://paymaster.biconomy.io/api/v2/84532/F7wyL1clz.75a64804-3e97-41fa-ba1e-33e98c2cc703";
const baseSepoliaUSDC = "0x036cbd53842c5426634e7929541ec2318f3dcf7e";
const recipientAddress = "0x0000000000000000000000000000000000000000";
 
const paymaster = createBicoPaymasterClient({
    transport: http(paymasterUrl)
})
 
const paymasterContext = toBiconomyTokenPaymasterContext({
    feeTokenAddress: baseSepoliaUSDC
})
 
const nexusClient = await createSmartAccountClient({
    signer: account,
    chain: baseSepolia,
    transport: http(),
    bundlerTransport: http(bundlerUrl),
    paymaster,
    paymasterContext
}); 
 
const userOp = await nexusClient.prepareUserOperation({
    calls: [
        {
            to: recipientAddress,
            value: 1n,
            chain: baseSepolia
        }
    ]
})
const quote = await paymaster.getTokenPaymasterQuotes({ userOp, tokenList:[baseSepoliaUSDC] })
const usdcFeeAmount = parseUnits(
    quote.feeQuotes[0].maxGasFee.toString(),
    quote.feeQuotes[0].decimal
)

Sending a transaction and paying the gas with USDC

import { createSmartAccountClient, createBicoPaymasterClient, toBiconomyTokenPaymasterContext } from "@biconomy/sdk";
import { baseSepolia } from "viem/chains"; 
import { http, parseEther, parseUnits } from "viem";
import { privateKeyToAccount, generatePrivateKey } from "viem/accounts";
 
const privateKey = generatePrivateKey();
const account = privateKeyToAccount(`${privateKey}`);
 
const bundlerUrl = "https://bundler.biconomy.io/api/v3/84532/nJPK7B3ru.dd7f7861-190d-41bd-af80-6877f74b8f44";
const paymasterUrl = "https://paymaster.biconomy.io/api/v2/84532/F7wyL1clz.75a64804-3e97-41fa-ba1e-33e98c2cc703";
const baseSepoliaUSDC = "0x036cbd53842c5426634e7929541ec2318f3dcf7e";
const recipientAddress = "0x0000000000000000000000000000000000000000";
 
const paymaster = createBicoPaymasterClient({
    transport: http(paymasterUrl)
})
 
const paymasterContext = toBiconomyTokenPaymasterContext({
    feeTokenAddress: baseSepoliaUSDC
})
 
const nexusClient = await createSmartAccountClient({
    signer: account,
    chain: baseSepolia,
    transport: http(),
    bundlerTransport: http(bundlerUrl),
    paymaster,
    paymasterContext
}); 
 
const userOp = await nexusClient.prepareUserOperation({
    calls: [
        {
            to: recipientAddress,
            value: 1n,
            data: "0x",
        }
    ]
})
const quote = await paymaster.getTokenPaymasterQuotes({ userOp, tokenList:[baseSepoliaUSDC] })
const usdcFeeAmount = parseUnits(
    quote.feeQuotes[0].maxGasFee.toString(),
    quote.feeQuotes[0].decimal
)
 
const userOpHash = await nexusClient.sendTokenPaymasterUserOp({
    calls: [
        {
            to: recipientAddress,
            value: 1n,
            data: "0x"
        }
    ],
    feeTokenAddress: baseSepoliaUSDC,
    customApprovalAmount: usdcFeeAmount
})
const receipt = await nexusClient.waitForUserOperationReceipt({
    hash: userOpHash
})

For a complete list of supported ERC20 tokens that can be used for gas payment, check out our supported tokens documentation.

By following these steps, you have successfully calculated the USDC fee amount and enforced a custom approval amount for a transaction using the SDK. For more advanced features and options, refer to the official documentation.

See sendTransaction Docs