Customize Shopify Checkout Page

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   I would like to ask about the customizing for the shopify checkout page. I already know about some of the answer such as you are not upgrade the shopify plus plan, u cannot customize your checkout page.But my question and requirement is here.
  I want to customize  two payment options as disable and enable according to the condition at checkout page shopify. I already tried with two methods.But not ok yet.
1) added additional scripts at checkout settings.(Question: this script is happening after complete order, but I want to run these script at checkout page payment section before complete order.) 
2)I tried with storefrontapi for fetching existing user's checkout with js-sdk, but in there what the question is,
    how do I retrieve existing checkoutId from store .and then replacing in the below code snippets.

For example 

client.checkout.fetch(checkoutId).then((checkout) => {
  // Do something with the checkout