Shipping Cost Calculator Service

Problem/Feature Description

A logistics company needs a service to calculate shipping costs for packages. The business rules are: standard shipping is free above a weight threshold, express shipping has a per-kg surcharge with a minimum charge, and overnight shipping has a flat rate plus a fuel surcharge percentage. The team is expanding the shipping options and the code currently has magic numbers scattered throughout — the same weight limit appears in four different places, and the surcharge percentage is buried as a literal in a conditional. This has led to bugs when one occurrence was updated and another was not.

Additionally, the team needs a standalone validator that checks whether a package's dimensions are within allowed shipping limits. This validator will be used by both the shipping calculator and a separate customs declaration service, so it should be self-contained and stateless.

Your task is to write the Ruby service layer for the shipping cost calculator. The rules should be cleanly encoded so that changing a business parameter requires updating exactly one place. Document the public API so that future developers understand the accepted parameters and return values without reading the implementation.

Output Specification

Produce the following files:

• app/services/<module>/shipping_cost_calculator.rb — main calculator service
• app/services/<module>/package_validator.rb — the stateless package dimension validator
• Any documentation or companion files you consider appropriate for the module
• spec/services/<mirrored_path>/shipping_cost_calculator_spec.rb — specs for standard, express, and overnight shipping, and for packages above/below the weight threshold

Do not create a full Rails application. Use plain Ruby with realistic values. The focus should be on the structure, documentation style, and how business rules are encoded — not whether the application runs end-to-end.