It seems that the only way to make sure whether everything has worked out the way you wanted it to during your purchase is by taking a look at the confirmation that comes afterwards. If you have selected a trade-in but you don’t see it as part of your order, then you have about 60 minutes to correct said order.
It might also be a good idea to record your screen while making the purchase, and have something to show to customer support just in case of this or other discrepancies. Google is no stranger to such bugs in the past, so it wouldn’t hurt to have that extra bit of reassurance, especially for a pricey piece of tech.