Click & Send is cheaper to send items valued under $100 if you want to qualify for seller protection and possibly be eligible for up to $100 compensation for loss.
The following is based on the 500gm satchel prices. There are other C&S services (non-eBay branded satchels / boxes) where some of this information and pricing won't apply).
The pre-paid satchels can be bought in bulk, with the best discount at 100 satchels or more in one purchase, making the cheapest you can get them for $6.48. They have a tracking number, but that's tracking only and generally, to qualify for PayPal seller protection, you have to register them at additional cost (standard cost $3.20, minimum $2.70 if purchased in bulk). That makes them cost a minimum of $9.18 and maximum of $10.40 to be equivalent to the Click & Send signature on delivery service.
C&S signature on deliver will cost $7.20 for the service ($6.20 basic service plus $1 for optional signature on delivery), plus satchel and label printing costs (that's around 20c, give or take depending on the quantity of satchels purcased and ink costs), so a total of say $7.50 and a difference of between $1.68 at the cheapest satchel / registered cost, and $2.90 if standard prices for those are paid.
Both of these options qualify for up to $100 compensation and provide the documentation required for PayPal's seller protection - the one advantage a normal satchel + registered has is that you can insure the contents for an amount above $100, which you can't do with an eBay satchel.
