winner want cancel

a person just won my item , now he wants to cancel as the postage i quoted wasnt enough to send to him in wa so put the correct postage , it was only another $4.70 as i quoted only $12.00 i usally put the postage is a quote only but on this sale i forgot to put this

this is the only problem i have on ebay and thats the postage costs

this is in a satchel addressed to him

so now what do i do? i said i wasnt happy about this and pointed out it was only another $4.70 to send this too WA, WHEN I DO SELL SOMETHING WHAT POSTAGE DO I QUOTE? as every state is different

i dont sell to overseas now as people complain about the postage from Australia people want you to post items really cheap?