Simple Delivery only approximates the weight, so it's supposed to be OK if the item weighs more than it says on the label. eBay have said:
'If you use the recommended parcel size and your item exceeds the maximum size and weight of the label, you won’t be charged any additional costs. You can still use the Simple Delivery postage label. The courier will accept your package providing it remains within the maximum weight and dimensions for their service.'
The alternative would be to cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
