Compliance issue with per unit invoicing

We currently use Payment plans to generate automatic invoices for almost all of our full qualification enrolments. 

When a student enrols, we have an internal unit added to their enrolment so that they can access their LLN assessment. This unit is not chargeable as it is a pre-req requirement before they can proceed with training.

Unfortunately, when we run an automation to create invoices, this non-chargeable unit is actually added to the invoice at the same rate the accredited units are charged. This makes a compliance issue as we are only meant to charge the published fee.

example Certificate 3 Guarantee students are charged a fee per unit. Instead of being charged for 15 units, they end up being charged for 16.

We need a way to flag a unit within a course or qualification as not chargeable or fee-free so when finance models that have per unit pricing are used with a payment plan, the invoices can be automatically generated without overcharging the student.