Simple Delivery only approximates the weight, so it's supposed to be OK if the item weighs more than it says on the label. However parcels are still refused so it's usually better 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.
The alternative would be to buy and pay for the correct postage yourself. To add that tracking number you still need to generate the original prepaid label. Then add the new tracking via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That needs to be done on a desktop browser.
