Holvi API, see https://medium.com/@lcx_at/using-the-holvi-api-with-ruby-4838f2499714 for details

holvi_api.rb

# encoding: UTF-8
class Shop
  SHOP_CONFIG = YAML.load_file("#{Rails.root}/config/holvi_config.yml")[Rails.env]

  def self.get_shop_url
    holvi_request = {}
    holvi_request["pool"]         = SHOP_CONFIG["pool"]
    holvi_request["purchases"]    = [{"product"=>holvi_product_id}]
    holvi_request["firstname"]    = firstname
    holvi_request["lastname"]     = lastname
    holvi_request["email"]        = email
    holvi_request["cancel_url"]   = SHOP_CONFIG["cancel_url"]
    holvi_request["success_url"]  = SHOP_CONFIG["success_url"]
    holvi_request["failure_url"]  = SHOP_CONFIG["failure_url"]

    holvi_request["country"]      = "Österreich"

    uri          = URI.parse("** API URL **")
    http         = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true

    request                  = Net::HTTP::Post.new(uri.request_uri)
    request["Content-Type"]  = "application/json"
    request["Authorization"] = "Token #{SHOP_CONFIG["token"]}"
    request.body             = holvi_request.to_json

    response = http.request(request)
    return false unless response.code=="201"

    JSON.parse(response.body)
  end

  def self.check_details(details_url)
    uri          = URI.parse(details_url)
    http         = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true

    request                  = Net::HTTP::Get.new(uri.request_uri)
    request["Content-Type"]  = "application/json"
    request["Authorization"] = "Token #{SHOP_CONFIG["token"]}"

    response = http.request(request)
    return false if response.code!="200"
    JSON.parse(response.body)
  end
end

Is this still working?! Holvi Support told me currently they don't have any API implemented?! Is there any reference-contact at holvi.com to ask for it?
Anyone knows a PHP/Composer implementation for this?