adding "available since" to invoice or order summary


on a product level one can add the date since when the product will be available.
The variable is: avail_since

On the product page this information appears with variable avail_date

However this variable does not exist in the notification variables, which is a little bit tricky for preordered products.

What would be the easiest way to add in the order summary or order invoice the date since the product will be available?

I have tried already product_avail_since and product_avail_date and p.avail_since and o.avail_since but it is not working and my IT knowledge is quite limited.

I would appreciate a lot your help.

Thank you very much in adavnce and best regards