I use C&S now for all parcels except the BX boxes (slight difference in price on the 500g flat rate satchels, they're 5c more), and what I often do is use the "add items" function to pull the ebay address data, then remove the items so that when the label is paid for, the info doesn't get sent back to eBay (the number isn't uploaded, and the order isn't marked as posted). 

Once it's been posted, I then add the number manually.