Manipulations total order in checkout

Shopify Partner
6 0 3

Hi, I want to create public app which is can manipulate amount of the total order. 

for example when total order IDR 270.000, I want create new variable 3 random digits number to decrease total order  to 269.346.

How is't possible? Should I create with Checkout API or Storefront API or something ?

some reference tell me I can't do anything in checkout page, except I'm on shopify plus. Is't correct?