The message is self-explanatory. There are certain situations where you cannot send a combined invoice and items have to be shipped separately. Can be anything from an eBay International Shipping (eIS) order to an Authenticity Guarantee (AG) shipment.
Best offers and auctions requiring autopay are not eligible for combined invoices, though they can be shipped together and you can refund the buyer the difference after they pay if you want. That's only if the items can be shipped together and aren't blocked by one of the other restrictions (not eIS, AG, etc).
