Thanks for the explanation
Tried the [subscription_id] idea but, as you thought, it didn't work.
Anyway, for the next version I would like to suggest some additional substitution fields, like that kind of information.
Using that id or other ones would be nice when we want the member to show a confirmation of his payment or just make it easier to check our own list. In this case they pay for something that they will use at a meeting later this year.
Again, thanks!