Please take care of the copy paste glitches. Billomat::Actions::Complete all over the place, correct to Billomat::Actions::Correct.

Conventionally this helper should be named out_of_bound? as it returns a boolean.

    # @param [Hash] paging_data The response from get_paging_data
    # @return [Boolean] whenever the given paging data is out of bound or not
    def self.out_of_bound?(paging_data)

Conventionally Ruby methods do not prefix get_ or set_ - just name it paging_data.

This method/design is not robust. Unfortunately we do not require/added ActiveSupport as runtime dependency - so we do not have String#pluralize, but it would be better to return the fully processed input string instead of returning a string suffix.

For future ActiveSupport addition it would be great if you could refactor this like so:

def self.pluralized_resource(name)
  # NOTE: When ActiveSupport gets included convert this to +String.pluralize+.

  return 'taxes' if name == 'tax'

  "#{name}s"
end

Then call it like this later:

resp[pluralized_resource(name)]['@total'].to_i

It is also not required prefix Billomat::Utils here, inside the same file to use a static method.

Since there's already resource_name, why not handle it within the model(s), e.g.:

module Billomat
  module Models
    class Tax < Base
      def self.base_path
        '/taxes'
      end

      def self.base_name
        'taxes'
      end

      def self.resource_name
        'tax'
      end
    end
  end
end

base_path is so generic that it can probably be moved into Billomat::Models::Base:

def self.base_path
  "/#{base_name}"
end

This way, you'd have:

resp[@resource.base_name]['@total'].to_i

# or

resp[@resource.base_name][@resource.resource_name]

I'm fine with this, too!

A mapped object with method access would be nicer to read/use at the clients.

article_tags = Billomat::Models::ArticleTag.paged_list(page = 2, per_page = 10)

pp article_tags.data
# => [Array<Billomat::Models::ArticleTag>]

pp article_tags.page
# => Int
pp article_tags.per_page
# => Int
pp article_tags.total
# => Int