// First retrieve the gas token prices
const gastokenPrices = await fetchGasTokenPrices();

// Build your TX
const calls: Call[] = [
  // ...
];

// Estimate the gas cost as you want
// Example: 
const fees = await account.estimateInvokeFee(calls, { blockIdentifier: 'latest', skipValidate: true })
const estimatedGasFees = fees.overall_fee

// Make your user choose the gas token
const gasTokenAddress = gastokenPrices[0].tokenAddress

// Execute the calls
await executeCalls(account, calls, { gasTokenAddress, gasTokenPrices, estimatedGasFees })