How do I apply a Verizon Apple One Family perk discount to an existing Apple One Premier subscription?

I have Verizon's Apple One Family perk. This includes 200GB of storage. I want to upgrade to Apple One Family Premier and I want to know the cost difference.

Apple Website:

Apple One Family: $25.95

Apple One Premier: $37.95

Verizon Perk:

Apple One Family: $20

Since Verizon offers the Family plan at a discount, what is the cost difference if upgrading Apple One from the Family plan to the Premier plan?

Thanks!