If you link your eBay ID to C&S, you will be able to get all the buyers' details filled into the relevant fields with a few clicks. The tracking number should also automatically be inserted into your and the buyers copies in sold and purchased.

To get the tracking number now, go to History in C&S. It is the item number. You can also opt to have tracking event advice sent directly to the buyer. I would caution against dropping them in a red box. Until they're scanned, you have no proof of postage.

After you pay, you select what you want to print. One of the options apart from address labels is Tax Invoice.