If your customer cannot pay the deposit upfront with you, there is a way to prompt them to pay it themselves from their Customer Portal.
For this you would use the Portal Payment section in the Confirmation Window. (see highlighted in RED below.)
You will just enter the amount you need the customer to pay and they will have to pay that full amount when they enter their card information into the portal.
**You will not have to enter anything here if you already collected the deposit**
When the customer sees their customer portal, they will see a button where they can pay this that says "Pay Deposit".
You'll see in the image below that they can either put in new card information or they can select from any cards they may have on file with you.