Automatic fixed charge for customer(No matter quantity)

Hi experts,

Due to a credit issue, we are going to start to charge a fixed quantity of 10 USD to every retail customer, so we need to know how to determine an automatic condition to do this.

We started to assign calculation type "B" and marked as header and item condition, but if we change the quantity at the delivery the fixed amount change. So we need to keep this condition unmodified until the billing document.

Please your comments.