I agree he was rude... so long as you are sure your weren't engaging in cell yell*. Some people truly raise the volume of their voice by 6-25 notches for no reason other then being on a cell phone. I've literally witnessed people having a normal conversation or chat with someone, then they pick up their cell and THEY SUDDENLY ARE CHATTING VERY LOUDLY, then they hang up, and they resume normal volume. I honestly don't think people realize they do it, but it doesn't make it any less annoying. And convenience stores are small confined spaces, so yelling can be uncomfortable for those around it.
*I'm also presuming you were not engaging in inappropriate private conversation, like about bowel movements or graphic sex or gristly amputations etc since you were discussing travel. I think those topics, in a small convenience store, would be a legitimate reason for someone to politely ask you be more private about your conversation as well.