Cancelling a sale

hi,

can I cancel a sale after a buyer has paid? The buyer has questioned my postage prices continually, ie the items weigh 700g and he writes to me that he can't believe the 2 items weigh over 500g even though I informed him of the price and weight in the first instance. He has continually asked me to drop the price from $13 (3kg satchel) to $11. He says this is all he pays in SA and that Qld prices must be more expensive??? He paid the full amount. So in my haste and bad taste left due to his complaining have shoved it in a 500g satchel, hoping it will get through. I refunded him $5 and now he is complaining about that. I'd really like to cancel the whole sale. can i just refund his money through paypal? I know he will leave bad feedback anyway.

Thanks