If the mistake happened when eBay imported Simple Delivery to your listings, then the courier is supposed to accept it regardless.
If this is your mistake and the Buyer paid (like on your auctioned clothing) then you could proceed one of two ways:
- pay out of pocket for your own Tracked service, and never tell the Buyer you made a mistake
- apologise and explain to the Buyer and ask if they would like you to cancel the sale and relist with correct postage. Good sellers would lower the item cost so that the Buyer doesn't lose out.
If you choose to not use the Buyer's SD label, they will be refunded automatically within 30 days. You should open their SD label to get their address. This will also mark the item dispatched. And here is where you also add your new Tracking Number.
Don't worry, SD gets a lot easier after your first few sales! It is actually better because it has better protection for you against lost/damaged items and the feedback associated with them. Also it automatically insures for up to £750.