Welcome to the community @Vogue
For a placed order event, the flow trigger is ‘placed order’. If you have only a selection of customisable products, you’d need to filter for those.
As this is a transactional email relating to their order, you need to be able to send it irrespective of the contact’s subscription status and also not include an unsubscribe link. To do all that, you’d need to create your flow/email and then contact Klaviyo to approve it as transactional i.e it only contains content relating to their order and does not include any marketing content. More info on that here: https://help.klaviyo.com/hc/en-us/articles/360003165732
Does that answer your question?
Regards
Andy