Empty cart message

Hi, I have a calendar where people need to book 4hours in advance. When they want to book closer to the date, they can get as far as the (woocommerce) cart before they get the 'cart is empty' message.

This is confusing for people. They expect a clear message that you can not book this close in advance, or it should not be possible to select that date in the calendar

This does not happen to all people... very strange

UPDATE

This only happens when the booking form is embedded in an iframe :-( I suppose iframe does not work well with woo checkout