Adding Validation to Checkout Forms would be great

I have a customer email me after their attempt to order was met with an error message.
“Sorry. We can’t ship your order to the address provided.”
I had to send them to the old website to complete their order.

So looking into what it might have been I have to these hypothesis;

  1. Their cart had a mix of items that require shipping, and do not require shipping. This assumption is because the product that does not require shipping was added to the store in coincidence with this error.
  • I have toggled them to “requires shipping”. I don’t see how this leads to that error either way, but it won’t hurt to toggle that on since it’s a free item with free shipping.
  1. They entered some invalid information in the checkout form like an extra Zip Code digit.
  • I’ve seen I can add a limitation to the Zip Code field so it can only be 5 digits. I have no reason to think that the customer entered any information that was not copy+paste what they always have (they are a return customer from our older website)

Asking for anyone who has dealt with this error code to share their experiences troubleshooting it. The last thing I want to have a customer spend 20 minutes filling up their cart with PPE and get stuck.