app.rb

# Run with ruby app.rb

require 'sinatra/base'
require 'httparty'
require 'pry'

class OauthApp < Sinatra::Base
  # Dev local API Client Credentials
  CLIENT_ID  = 'foo'
  SHARED_SECRET = 'bar'
  SHOP = 'shop.myshopify.com'

  def initialize
    @@access_tokens = {}
    super
  end

  get '/' do
    'Foobar'
  end

  get '/oauth/authorize' do
    shop_domain = "#{SHOP}.myshopify.com"
    scope = 'write_delivery_profile_settings'
    redirect_uri = 'tunnel'
    state = 'foobar'

    redirect("https://#{shop_domain}/admin/oauth/authorize?client_id=#{CLIENT_ID}&scope=#{scope}&redirect_uri=#{redirect_uri}&state=#{state}")
  end

  get '/oauth/access_token' do 
    puts "State: #{params['state']}"

    payload = {
      code: params['code'],
      client_id: CLIENT_ID,
      client_secret: SHARED_SECRET
    }

    response = HTTParty.post("https://#{SHOP}.myshopify.com/admin/oauth/access_token", body: payload)

    @@access_tokens["#{SHOP}"] = response['access_token']

    "Access token acquired: #{@@access_tokens[SHOP]}"
  end

  get '/activate_charge' do
    # Activate charge
    response = HTTParty.post("https://#{SHOP}/admin/api/2019-07/application_charges/440467478/activate.json")
    "Application charge created: #{response.inspect}"
  end
end

OauthApp.start!

Gemfile

# frozen_string_literal: true

source "https://rubygems.org"

gem "sinatra"
gem "httparty"
gem "pry"