This is the half of the app that has nothing to do with the COD form. It works inside the ordinary Shopify checkout and links one or more shipping rates to your COD payment method. Pick any of those rates and every other payment method is hidden, so card, Shop Pay and PayPal disappear and Cash on Delivery is the only way to pay. Pick a rate that is not on the list and Cash on Delivery disappears instead. Open Shopify Checkout → Activation, which has a three step guide.
Step 1: Create the COD shipping rate in Shopify
The first step is Select Shipping Rate for COD. Its instructions send you to Shopify's shipping and delivery settings: open the General shipping profile, add a shipping zone if you need one, then add a rate under your target zone, name it and set its price. Shopify does not let you add a fee to a payment method, so the price of this rate is where your cash on delivery surcharge goes. This step also has a Select delivery option picker: use it to choose the rate you just made. It takes more than one, so add every rate that should carry cash on delivery.
Step 2: Create the manual payment method
The second step is Enable Cash on Delivery Payment. In Shopify payment settings, scroll to manual payment methods, add one, choose Cash on Delivery (COD) and activate it. This step also has a Select payment method picker: use it to choose the Cash on Delivery method you just made.
Step 3: Activate
The third step is Enable COD in Checkout. Press Activate. Once it is on, the step reads "COD will be visible in checkout only for the selected shipping rates." and the button becomes Disable COD in Checkout. Two checkout customizations now appear in your Shopify settings, named Customize available payment methods based on selected shipping and Customize available delivery methods based on selected payment. Leave them switched on: they are how the rules take effect.
Step 4: Test in checkout
Go through your own checkout twice, once choosing a COD rate and once choosing another.
What to do next
Need finer control than one rate? Write a visibility rule on country, cart value, products or customer.
Renaming the rate or the payment method in Shopify breaks the link, so come back here and pick it again if you do.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.