Kath, it has been happening since before Christmas.

I was trying to purchase some small items from 2 different sellers in the UK who I have dealt with before and always had combined postage.

When I contacted them about the problem they both said that their settings had not changed and there was nothing they could do from their end.....it was either pay the full amount or don't buy.  As the postage was minimal (small, light items sent in an envelope) on the items I paid up in full but felt a bit annoyed as the items ended up being much the same price as I could get them for in Australia.