Like the old adage goes, "anything you say, can and will be used against you." You should definitely not respond unless and until you get a lawyer who can do the response for you. For now, your response is removing the clothing from your store, which I'm sure they've noticed.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
Do not write them back on your own. You need to retain counsel to represent you so that the response to this cease and desist letter does not prejudice your position. You need to take this seriously and move quickly to retain counsel, but once you retain counsel the 72 hour demand can be adjusted.
If you do not respond within 72 hours, the worst thing that could happen to you is that you might get sued. While you might prefer to avoid this, a cease and desist letter is only one step removed from a law suit, and the difference is not as great as you may think. You need to retain counsel immediately, and let your counsel handle the negotiations concerning how to resolve this matter.
You have taken proactive steps including removing the products in dispute and seeking a lawyer, so you are not ignoring them. The 72-hour deadline is probably an arbitrary one, and you should probably refrain from responding until you retain legal counsel. A few days is probably not going to make much difference. A few weeks may irritate the other side. If you find that it is taking longer than expected to find suitable legal counsel, and you feel that you cannot wait, then you may want to consider sending a short response that you are seeking to hire an attorney (and say nothing more substantive). In general, you are better off not responding directing and having an attorney handle this, but the decision is yours.
This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.