To print or save a PDF record of your sale, just head to your Selling / Orders page.
From there, click the order number next to the item you need a packing slip for.
Then, click "Packing Slip". This will give you a printable PDF that includes all the order details, which you can use for your own records or to include in the package you're sending to the buyer.
You can follow the same process for finding an invoice or packing slip for Reverb LP orders.