ebay has now added the ability to easily print a Coded Coupon either as a standalone package insert, or at the bottom of the new Packing Slip (which I believe is still only available through the Seller Hub>Orders> dropdown for each order: "Print Packing Slip and More")
So, if you are one of those sellers who doesn't use Packing Slips but wants to enclose a coded coupon, the standalone package insert works. If you already use a Packing Slip, then adding the coupon to the bottom of the sheet is the easiest way to handle this.
I just added one to a package going out tomorrow-----easy peasy. (First step, to be clear, is to actually create a Coded Coupon...then you can go about adding it)