In your "Apply to Each", you will want to see if you get use the SharePoint "Get Items" action to Filter to something unique about that entry. For example, if the Title is something unique to all of them, that will never change, then you would use that. But you will need a valid, unique key.
Then, if you receive an item from the "Get Items' - you know it already exists. If you don't, then you'll need to create it.
A lot of times what I'll do is use a Condition, and for the conditional check, I'll use the expression of length() (where you would put the output from Get Items in between the brackets), and then "is less than or equal to" 0. If that's TRUE (ie, there are 0 items in there), then you Create. If it is NOT true, then you update. And the ID will have come back through the Get Items, so you can then use the Update Item action to do so, and point at the ID.
Does that make sense? The unique key is going to be very important though. I am hoping maybe your Customer Name would be unique? But maybe it isn't... if not, you'll need to create a unique key.
