How can I set up transactional emails for long wait orders?

[atompacks]

Hi everyone, thanks in advance for the help. 

We sell customisable items with a variable waiting time of sometimes 3+ months. I need to be able to send transactional (not marketing) emails updating customers on their order and how long the wait will be. 

Ideally these emails would be triggered by a tag added to the shopify order. 

I've had a look at Klaviyo and a few others but I cant find an easy way to set this up. Shopify flow would work just fine if it allowed us to class these as transactional and not marketing. 

Any ideas would be greatly appreciated!

Regards,

Dexter