[tajen]

I'm the author of a product and in some situation I have subscribed former users to a one-off email. It's gray area since it was to notify them about a major release/change, it was written in my T&C and sign up form, but honestly it would feel legitimate if they marked my email as undesired/spam.

Out of 900 recipients, 1 marked it as spam, 18 unsubscribed. I believe people are very tolerant and they could use "spam & blame" a lot more.

[linkregister]

I think your situation is different; you're not subscribing them to a newsletter. Your communication is reasonable for a typical business relationship. (and is protected under CAN-SPAM)