I'm not trying to be over vague here, but do the letters say they're from the IRS? Do you have any reason to know that they might be from someone other than the IRS? I ask this because there had been some fraud letters sent in the past claiming to be from the IRS stating that the tax
payer had a refund coming from a prior year, but they needed to send in some $ for "processing" or some such thing, but to the best of my knowledge, these letters were NOTsent via certified mail, and the IRS did manage to track down some of these companies that were fraudulently using their letterheads, etc.
If I were you, I personally would pick up the letters if for no other reason than to find out what's in them. However, I understand your caution, so I suggest that if you don't want to just pick up the mail,go to the post office and explain that you're rather suspicious of these letters, and see if you can find out the address/phone # XXXXX the post office will be sending proof that you claimed the letters, and see if you can find the address on the web, and if there's a phone #, call it and see who answers.
I do know that the IRS has satellite offices all over the US, and they do NOT make these addresses available since the offices are not set up to help people in person. In fact, there was an office here in Kalamazoo, MI that they reported as being closed (which it was for walk in services) a full 3 years before they finally really did close it and move it elsewhere. It went from a full service IRS office to only dealing with tax payers on the phone, and by correspondence. I know the office is now empty, and even working in this field for 25 years, I have no idea where they moved it to.
So do what makes you comfortable, whether that means picking up the letters and reading them, or trying to track the sender via address/phone # XXXXX you decide to pick up the letters and you still have doubt about their authenticity, please come back here and let us help you. You may ask for me by name if you want, or you may just post another question and I'm sure a tax pro will pick it up. There's alot of talent on this site, and helping you figure out if this is a legitimate IRS letter or a fraud is, unfortunately, a question we deal with on a fairly regular basis.
I hope this helps.