// https://developer.apple.com/documentation/storekit/product/purchaseresult
let result = try await product.purchase()
switch result {
case .success(let verificationResult):
    switch verificationResult {
    case .verified(let transaction):
        // Give the user access to purchased content.
        ...
        // Complete the transaction after providing
        // the user access to the content.
        await transaction.finish()
        ...
    }
case .pending:
    // The purchase requires action from the customer. 
    // If the transaction completes, 
    // it's available through Transaction.updates.
    break
case .userCancelled:
    // The user canceled the purchase.
    break
@unknown default:
    break
}