Thanks all for your great input! I did as DG suggested and made a call based on FB etc... I actually got the buyers number from the invoice and sent a message and he ended up calling me so felt safe enough to do so. Everything turned out well and buyer has already left feedback for all three items so happy result all round!

In future though, I will add the suggestion from PJ, Lyndal, Go-Tazz and others on adding a note to my listings about combined postage and needing to wait for an invoice which I don't currently have on my listings.

I also refund significant postage differences without even taking into account the fees, but I think I will also do as others have suggested and put that in my listings too! Thanks for all the input, always helpful to hear how others approach things, especially in the changing world that eBay is becoming!