Free
1. A deposit product and an invoice
You add a second product called something like "50% deposit", price it at half the real price, and say in the description that the rest follows when the item ships. The buyer orders that one, and when the stock lands you write a Wix Invoice for the other half.
It costs nothing in money. What it breaks is the order. The order is for the placeholder, so your stock count and your reports show a deposit instead of the thing you sold, and every balance is an invoice you write and chase by hand.