Punchout Vendor

Hi,

We have a Punchout Vendor, where the shopping cart is eligible for free shipping if it is 10$, less than 10$, no free shipping, Our requirement is if the user submits a shopping cart less than 10$, it should not submit and instead it should show up a message saying that the Order did not meet the minimum requirements for free shipping.

Please advice what is the best way we can achieve this requirement.

Regards