Has anyone ever heard or (or used) the USPS Tracer Service as shown here? It is apparently a "pilot program" under testing at certain post offices. In essence, it's a delivery confirmation service but for first class mail (as delivery confirmation is not available for first class mail). The cost of $1.99 for 5 or $2.99 for 10 "bar codes" comes out to a fee per cover of 40 cents and 30 cents respectively.

I imagine cover collectors will be interested in acquiring the USPS Tracer Service bar coded covers, especially if this is only to be a "pilot" program.