Easypost causing orders to go through with no shipping fee

Hi,

I am having problems with orders going through without being charged shipping. I am using the printful plugin and have been going backwards and forwards with them for the last month. They say there is nothing wrong with their plugin and that this problem is being caused by Easypost. Their email reads:

I’ve spoken with our developers today about your store’s orders. Easypost runs in the background and aids in returning shipping rates to Printful (and by extension, your store) when an order is being placed. On occasion Easypost fails to return shipping rates, although it should every time. In your store, you’ll need to alter your settings to prevent customers from completing orders that don’t have shipping selected. This shouldn’t cause problems in most cases. You’ll need to speak with Woocommerce’s support team if you need assistance updating this setting.

Can anyone at woo or anyone else who had this problem help?

Thanks in advance.