He is only sending letter to people in the States where US law applys. He can send letters to foreign sellers, they just ignore those letters.
This whole thing is about business. He is not telling you that you can not sell the canopies, just that you can not use two word together. Change the wording, attribute the owners of the trademarks. that is all that is needed. Do not let this get you down. It is a small bump in the road. Other have said it , better than I, it is about the words, and using those words to market products.
Your technique, your openness are spectacular. Please do not let this get you down.