Hello @DevinEngle 

I would keep the things simple.

If the customer at cash register states they want to join the email list and provides their email then Send the Profile to Klaviyo as Subscribed and Add them to a list and I will add them to a single Opt-in List.

If a customer is ever at the cash register and says they no longer want to receive emails, should we push that to Klaviyo as unsubscribed or as suppressed? 

This is something you have to make a business decision on. 

Here are few things I would consider

• Do you want to target them on Web popups to get their consent?
• Are they sharing additional information like Phone numbers and want to receive SMS only?
• Are they opting out from Email only? But you can still target them on Social.

If a customer is ever at the cash register and says they are not getting the email but really want to (and they perhaps unsubscribed in the past -- our clients report this is a frequent occurrence), if we push them as subscribed

You can use “Unsuppress API” to Unsuppress them if they were suppressed manually and then Subscribe them back.

If you have multiple sources to opt-out and opt-in, I would use additional attributes to capture timing of Subscription/Subscription and Sources of each action.