If you send it regular mail you won't be able to prove that you sent it in court. The court will accept a statement from the plaintiff or his attorney claiming that they sent you certain letters because they are a business and courts generally consider that business operate according to a set of rules governing everything they do. But Courts generally won't believe statements from a debtor unless they can back up everything they say with concrete proof.
Instead of RRR since it is only being used for proof of service, why not send it regular mail with proof of mailing? Much cheaper than RRR and the PO gives you a receipt to "prove" that you mailed it and to whom.
Again, you want "evidence" that they received it, "proof of mailing" does not equate to "proof of receipt". Even with CMRRR, it is strongly advisable to have the CMRRR number on the letter itslef to evidence what was actually sent to the business.
I finally did send it 'delivery confirmation'. This ended up costing me almost as much as the CMRRR so I should have just went ahead and did it that way anyway. I'm not going to ponder over it anymore. I've decided just to send everything CMRRR to be on the safe side. IN the long run, I'm sure it will save me much more than I ever will spend.
It is worth the few extra dollars, there have been times when I was told that a letter was not "received", and suddenly everything changed when I asked who the signature person was!