I’m new to Ebay and I don’t have the Paypal yet. The seller and I have agreed to pay it by cash by doing the postal order. I’ve searched on the internet how to do it, and it says I have to go to the Post Office and ask for money order. And then what’s next?

I’m living in Canada. And if the seller gave me the price of the item in British currency, do I have to exchange the Canadian dollar to pounds in the bank? Or I could send the cash in dollars?