Ernest, thank you again for your quick reply!
I've tried as you suggested (see SS attached). I've attached a screenshot (SS1) of the posts and what I've tagged them and then attached the screenshot of the results (or you can go to https://www.travelhoney.com/iceland/). You'll see that one of the posts is categorized as just Iceland, but we could just check all the subcategories for that if it's helpful to not have a parent category. With this set-up, it appears that if you only check Reykjavik, it only pulls up the post that only is categorized as only Reykjavik (SS2), it doesn't pull up the posts that are categorized as Reykjavik and Golden Circle (you need to check both those boxes for those posts to appear). I then thought I could check At Least One of the Terms instead, because if at least one of the categories is checked, we want the post to show up – but no matter what box I check, all the posts appear (SS3). Any other thoughts or workarounds?
Adding the shortcode seems like it could work, but I don't know where to add that code, because we're using a sidebar (SS4). I'm guessing that issue doesn't have to do with your plugin, but if you have any suggestions, let me know!
Thank you again!