The product rate code is normally a code that National automatically puts in for you. Most of the time, you don't save any money putting it in yourself, in my experience over the years. So I agree with the previous poster. Try with and without it and I'll bet you'll get a better code without it.
Usually, if you click on an offer at their website, that is when they automatically plug in the code for that offer, if you qualify for it (it's the right timeframe for that offer). So if you take that code and plug it in for another timeframe that is not part of the offer, that is when they'll probably be no savings for you and maybe even an increase in the price.
I never plug in the code. I only do the contract code and the coupon code.