Of course, I keep thinking about my own items (which have a postage cost). 

Hmm, seems like a weird stipulation to have, though, if free post applies regardless - I wonder if they meant it for the express post option instead, since that can have a cost? At least that way, you can charge your regular rate of express and have a minimal cost for extra items. 

I doubt you'd have anyone query such a small additional cost, if they even notice. It might look a little bit weird having no extra cost for the free post option and a small charge for express, but it's not like postage to AP is the only cost you have for shipping and most people wouldn't bother querying an extra 5c. You could even try it with 1c.