I can see the new shopify remix update does not has a server file. How can i authenticate customer from shopify if i have my own server.

Hi guys i am creating a cart display apply and removal app. I have found a way to apply discount But i can't remove the discount in cart . I am using this url to fetch and make that happen."${shop-domain}/discount/="Can u guys help me with it

I have a app which create custom shipping option in check out page using carrier service api but i requires higher plan for my client so is there any way i can recreate this function without carrier service.