I've heard that if the value is less than $20 US, then an international customer probably won't have to pay any duty on it. Having read the comments here though, that does seem to vary by country.
I only put down the value of the actual item. Postage shouldn't be included in that, and if it is (and you're pretty sure of it), I'd be letting the seller know just so they don't continue to do that in the future. :-)