I agree with
@whateve.
I empathize with the shock you must have felt in getting a "reminder" (as ebay calls it) but many sellers, especially those with lots of listings, have the ebay unpaid item assistant set to automatically open cases. They likely don't even pay attention nor look at your history or feedback.
It certainly isn't worthy of a feedback ding as long as the item you receive is as described. The seller didn't do anything wrong and was completely within the boundaries of the policies set by ebay.
There's nothing stopping you from (politely) apologizing to the seller for not paying immediately immediately and explaining that you were away.
I have another comment. Many Japanese sellers work with each other and/or have more than one ID on which they sell the same items. So although this is a low-feedback seller, he may have another hundreds (or thousands) of listings under another ID. And if that's the case, contacting every non-payer before the UID dispute opens automatically would be impractical or impossible.
As
@whateve said, if you pay before the case closes in 4 days, you won't get a strike or any other type of ding.