custom_calculator.rb

## app/models/custom_calculator.rb

class CustomCalculator <  Spree::Calculator
      
  def self.description
    "foobar"
  end

  def compute(object=nil)
    return 10.00
  end
end

spree.rb

# config/initializers/spree.rb
Spree.config do |config|
  # Example:
  # Uncomment to override the default site name.
  # config.site_name = "Spree Demo Site"
end

Spree.user_class = "Spree::User"

## First I used:
#
# Rails.application.config.spree.calculators.shipping_methods << CustomCalculator
#
# But rails server throws Error: config/initializers/spree.rb:22:in `<top (required)>': undefined method `<<' for nil:NilClass (NoMethodError)


## Then I used:
config = Rails.application.config

config.after_initialize do
  config.spree.calculators.shipping_methods << CustomCalculator
end
## rails server starts but Calculator is not visivle