Add a delivery fee if order is under $20

My son is selling banana bread to raise money for a Boy Scouts summer trip. I created a quick app for him in Knack to take orders. We are offering free delivery for orders over $20, anything under $20 I want to add a $5 delivery fee.
We have 2 types of bread, banana and banana w/chocolate chips. In the order form I created a dropdown menu to select how many loaves of bread for each type. Then I have a formula that adds them together and gives a sub-total.
How can I have it check to see if that subtotal is less than $20, and if so add a $5 fee?
I’ve tried a few different ways but none of them seem to work. It either always adds the fee or never adds the fee.

You need 3 tables:
Bread Line Items

Bread Line Items Extended field is an equation:

Bread Orders Total Line Items Field:

Bread Orders Delivery Fee Field w. Conditional Rule:

Bread Orders Order Total equation field:

Bread Records:

Bread Line Item Records:

Bread Orders Records:

Note: On your actual page w. Line items, you’ll need an Update Rule to fill in the Bread Cost from the Bread Table.

That’s all she wrote!