Cheryl, I hope this message finds you well, present circumstances excluded. I am a licensed IP attorney with over a decade of legal experience.
This is an issue that is becoming more and more common. We have companies that market product on-line, which makes it easier to search for what could be infringement and we have law firms willing to pursue these allegations of infringement (and even search them out).
As you likely know, infringement in this context is determined by whether or not there is a likelihood of confusion on the part of the consumer. In the present case, as with all copyrights, by you creating your product, you have a common law copyright. If they have filed for federal copyright protection, then there is a presumption in their favor that they own what they claim to be theirs. However, they still must put forth evidence to show that you are indeed infringing on their idea. This means expensive litigation and expert fees (it means that for you too if they sue, unfortunately).
Based on what you have shared with me, I don't think you are infringing on their copyright. That said, defending frivolous litigation is still expensive. Therefore, we must also think of this in business terms. You must ask yourself, am I making enough money off of this design to justify potential litigation that could cost me north of $100K to defend?
If the answer to that question is no, then you should remove it from your product line. If the answer is yes, then you should respond to them that you are not infringing on their copyright and therefore you are going to proceed in making and selling this product, to which you have a common law copyright yourself.
If you decide to remove it, send them a certified letter with a return receipt which specifically states that while you feel as though you are not infringing, and that they may even be infringing your common law copyright, you are going to remove this from your line as a token of good faith.
I know that this is frustrating. However, under the circumstances, these are the options at your disposal.
Let me know if you have any other questions or comments.
Please also rate my answer positively (three or more stars) so I can receive credit for my work.
Best wishes going forward!