Best way so that you can get the correct Simple Delivery label and therefore have the item tracked and marked as dispatched is to cancel the order with the buyer's agreement,  and relist it with the correct postage price.

Send the buyer a message explaining the above that you will be cancelling the order citing Buyer Requests to Cancel ( this then does not affect your seller performance),  this fully refunds them and the item will be relisted with the correct postage for the weight of the item.

A refund  for the unused SD label,  is sent to whoever paid for the SD label:

• If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days

• If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling.

@rob4073