How to add a custom calculator for shipping or taxes to Spree

README.md

1. Add initializer to config/application.rb
2. Add new class to app/models/spree/calculators/[class_name].rb
3. Add logic to class file

Class must have the following methods:

def self.description
  "Custom FlexiRate"
end

def compute(line_items)
  13.00
end

application.rb

# config/application.rb

require File.expand_path('../boot', __FILE__)

require 'rails/all'

if defined?(Bundler)
  # If you precompile assets before deploying to production, use this line
  Bundler.require(*Rails.groups(:assets => %w(development test)))
  # If you want your assets lazily compiled in production, use this line
  # Bundler.require(:default, :assets, Rails.env)
end

module Peelstar
  class Application < Rails::Application

    ...
    
    # Add a new calculator for custom shipping or tax options
    initializer 'spree.register.calculators' do |app|
      app.config.spree.calculators.shipping_methods << Spree::Calculator::CustomFlexiRate
      # app.config.spree.calculators.tax_rates << Spree::Calculator::CustomTaxCalculator
    end
  end
end

custom_flexi_rate.rb

# app/models/spree/calculators/custom_flexi_rate.rb

module Spree
  class Calculator::CustomFlexiRate < Calculator
    def self.description
      "Custom FlexiRate"
    end

    def compute(line_items)
      13.00
    end
  end
end

Instead of this path:

app/models/spree/calculators/custom_flexi_rate.rb

It should be this path:

app/models/spree/calculator/custom_flexi_rate.rb

Notice calculators (plural) -> calculator (singular).