Why you may not be recieving "transactional" platform notifications
There may be a few different reasons why you are not recieving notifications, but I am going to strictly talk about the Listing Type. <ListingType>StoresFixedPrice</ListingType> is technically no longer valid and is not supported. So if you are listing your items using this listing type you may be missing notifications such as AuctionCheckoutComplete. You want to ensure you are listing with <ListingType>FixedPriceItem</ListingType>. You can double check your item by making a GetItem API call and checking the listing type. If you do find your item has a "StoresFixedPrice" listing type, unfortunately <ListingType> is not an option and is an invalid field when revising an item. So you will have to end and relist the items.
Below are a few more articles on triaging platform notifications.
Answer Title: Triaging a Platform Notifications Problem
Answer Link: https://ebaydts.com/eBayKBDetails?KBid=706
Answer Title: SetNotificationPreferences returned Success, but I'm not receiving any notifications, and GetNotificationPreferences shows my ApplicationURL as 'http://dummy.com'
Answer Link: https://ebaydts.com/eBayKBDetails?KBid=354
Answer Title: How can I determine why I am missing some Platform Notifications?
Answer Link: https://ebaydts.com/eBayKBDetails?KBid=407