Hello @michellebiggs,
Great question!
Flows are an automated sequence that is set in motion by a specific trigger; in the case of a birthday flow, it would be when the date found in a contact’s profile property is reached. Because of this, there would not be a way to manually send a flow email to a customer without them automatically triggering the flow and receiving the corresponding emails.
Instead, I would suggest sending this email either as a campaign or to send this email to this contact individually. You find more details on how to create and send a one off email to a single recipient from the How to Email a Single Person Using Klaviyo Help Center article.
@Maria.ramos-esperanza also has a great explanation of the difference between Campaigns and Flows from the following Community post:
I hope this helps!
David