Hi @forman! Great questions. Rather than a flow, it sounds like a campaign would be the best way to send these messages. You’ll first want to create a segment of the appropriate audience. For example, you could identify customers from the past 2 days using this segment definition: 

Next, you can build your campaign to notify those customers of the delay, and choose your new segment as the send-to list. When you’re ready, send the campaign! 

These guides go through the process in more detail: 

https://help.klaviyo.com/hc/en-us/articles/115005054847-Guide-to-Creating-and-Sending-an-Email-Campaign

https://help.klaviyo.com/hc/en-us/articles/115005237908-Guide-to-Creating-Segments