According to the RAE dictionary, todavia and aún are indeed interchangable and that is how I use them in everyday life. Todavia is a bit less formal than aún and more common in spoken Spanish. On the other hand aun (no accent) means even:
I would give you 100 pounds and 200 even!
Te daria 200 libras y aun 200!
Your response was fine, Tom - in an ideal world, we’d have managed to be entirely consistent - so our bad - but at the same time, it’s not all bad for you to get occasional bits of exposure to patterns you’ll run into in the wild…