How to attach data to Orders, and show these additional data in Order Page for Admin and Customer?

[AhmadAtaya]

We are building a Drop-shipping plugin for a Gift-Cards website to publish it in Shopify Store.
When the Customer purchases one of the products that a merchant has already added to his website, the App calls an API to return the Serial Code and Serial Number, after that, it will attach Serial Code and Serial Number to the Order, so the merchant can see them in the Order's page (we need to do so, just to allow merchant to send them again to Customer incase a customer didn't receive his email), and also they should appear in the email that Shopify sends to the Customer.

My Question is: What is the suitable field to store these Serials? 
-We tried to use the GiftCard API, but it requires the App (our plugin) to be a custom/private Plugin (a plugin that cannot be published in Shopify's Store).
-Also we can't use the metadata field in Order API, since it won't be shown in the Order's page or in the Email that Shopify sends to the Customer after a transaction done.

So, any suggestions?