Mathematically speaking 0.000000000000000000000000000000001 is considered zero, even though it's really not.
I've seen so many of these terms floating around here that I have to react. This pisses me off. You could say "practically speaking", but most certainly not "mathematically speaking". Same goes for "mathematical certainty". Mathematical certainty is exactly 1, nothing less, making nothing in real life mathematically certain. Mathematics are exact, there is no room for such approximations. /rant
When you're only given a few short moments to explain an idea you don't need to drone on about how 0.0005 BTC is worth less than half a penny, so the fees are so low that they can be considered negligible.
Zero fees! Done!
If you've hooked the person enough to really look into it, they'll figure it out for themselves.
You could also say "Almost zero fees! Done!". This way no one will mark you as a liar later on.