I also make it very clear in my listings that I am happy to combine postage on items purchased within a 7 day period. I also ask buyers if purchasing more than one item, to please wait for my invoice before paying to receive combined postage cost.

They rarely do.

But I'm a nice person, so I refund them the overpayment on postage anyway. I won't be doing it from next month however, unless of course the buyers of multiple items actually wait for my invoice.