title: Craft CMS allows a zero sale when going direct to /checkout link
body: <p>I have a client that has a craft CMS site selling apartments
  and he has had a few sales coming in for a zero amount</p>
  
  <p>and we wasn't sure how this is happening as all apartment have a min 250.00 deposit scheme</p>
  
  <p>browsing the apartments is fine when you find one you go and book it with a deposit.</p>
  
  <p>but we have found out if you don't look at any apartments and in an incognito window just go directly to domain/checkout</p>
  
  <p>you are allowed to fil in a form and pay a zero amount. this is causing a problem as they are getting quite a few like this now.</p>
  
  <p>is there a way anyone knows how to stop access to /checkout if the value is a zero rate</p>