Quite often I get buyers who do not wait for, or create for themselves, a combined postage invoice when buying multiple items... this is quite tiresome for me as I then either send them postage or a partial refund to compensate as it is not fair to overcharge on postage. And I end up paying more in Paypal fees as well...
+ I do have a notice - in red - stating 'combine lots to save postage costs' in all my listings!
The billing/invoice screen needs updating to allow the buyer to continue without paying straight away.
Currently it is not so obvious.