Pay deposit at checkout on BigCommerce store

When the customer gets to the Payment section there should be an option at the top with a checkbox, asking them if they would like to pay a 20% deposit. If they check this option the order total is reduced to 20% of the total. The order summary should show how much of a deposit is been paid and the outstanding balance to be paid. They then choose their payment method Debit/Credit Card or Paypal and pay the 20%. The order comes into the back end marked as Awaiting Payment with the details of the deposit amount. We will manually collect the remaining 80%.
Please ask questions if you are unsure of what is required.