Over time, I've been updating my listings with advice on how to checkout multiple items to get combined postage discounts (using a mobile device), I also have it in the extra information in the P&P tab, plus it gets sent out on all invoices.....aaaand I include the info in my reply to anyone who asks about combined postage, on the off-chance it helps.
Chipping away at the block one speck of dust at a time. 
But yes, it's ridiculous and the above shouldn't be necessary - eBay cannot expect to be able to name themselves a "premier" shopping destination (as they're so fond of doing) if they make it difficult to purchase from the site itself, but very easy to purchase from other, more user-friendly sites via third party advertising. They pat themselves on the back for their "improvements" re: home page, collections etc, and do so little in the way of ensuring standard, necessary site functions actually do function.
It's maddening - I state that separate payments = separate packages in my TOS, but when someone buys five items and I can post all together for the same price as one, there's just no sense in insisting on posting them separately, and I've not had one person (if I do send a query) come back and say "no, I really want each item in its own padded bag." Not to mention continual refunds for excess postage puts my PP account at risk of being flagged / restricted. >_<
They really just need to to get rid of the "Buy It Now" button and only have "add to cart" (for fixed price listings), as well as ensure the cart functions properly (preferably without loading the cart when you click "add to cart", like every other half-decently-thought out commerce site).
