stuff.js

if (!checkoutDetails.finished)
  checkout.continue().then(finishCheckout);
else
  checkout.finish(...);
  

var checkout = new Checkout();
checkout
    .onEvent('shippingOptionChange', recalculateLineItems)
    .show()
    .then(finishCheckout);

// Returns line items with updated price/tax/etc based on the options that the user has selected in checkoutDetails.
function recalculateLineItems(checkoutDetails) {
    ...
}

// Requests the payment from the browser.
function finishCheckout(checkoutDetails) {
  checkout.finish(...);
}