No matter how flexible a site's shipping options are, there will be some sellers who can't do their postage exactly the way they want to, in an immediate payment required scenario, I mean - the best thing to do (if willing) is simplify it as much as possible. My advice won't work (or be accepted*) by everyone, but imagine for a moment that you had no choice, that eBay was abolishing the ability to invoice for a sale post-purchase (but pre-payment) tomorrow, and you pretty much either had to figure out a way to work with the options available, or end listings, how would you approach shipping then?
The last time I suggested thinking about re-evaluating shipping that way on the forum was a few years ago now, and it was met with a lot of pushback, and people seemed to take it personally for some reason, (*hence both the disclaimer and the long gap between saying it again 
), however I genuinely believe it can help to at least re-think the approach as a thought experiment if nothing else, but at the same time I am convinced the majority of buyers would prefer costs (and combined costs) not just stated up front, but calculable or predictable on multi-item purchases, even if it costs them slightly more on some occasions or on average, and even if a lot of people respond to this by saying "not this buyer!", lol.
