Little refactor

custom_fields.rb

class CustomFields
  def initialize
    @hash = {}
  end

  def []=(key, value)
    @hash[key] = value
  end

  def to_hash
    change_booleans
  end

  private

  def change_booleans
    new_hash = {}
    @hash.each do |k, v|
      if v.is_a?(TrueClass) || v.is_a?(FalseClass)
        new_hash[k] = v ? 'Yes' : 'No'
      else
        new_hash[k] = v
      end
    end

    return new_hash
  end
end

usage.rb

# Before
custom_fields = {}
custom_fields['Postcode'] = s.postcode
custom_fields['TestAlreadyBooked'] = s.test_booked ? 'Yes' : 'No'
custom_fields['PaidSubscriber'] = s.latest_subscription ? 'Yes' : 'No'

api(custom_fields)

# After
custom_fields = CustomFields.new
custom_fields['Postcode'] = s.postcode
custom_fields['TestAlreadyBooked'] = s.test_booked?
custom_fields['PaidSubscriber'] = s.latest_subscription?

api(custom_fields.to_h)