Thanks again for your ideas ... it all makes good sense.
I'm not a rep for them either but I was able to get it to work. Start over from the beginning by emptying your cart. Then just add any three packs of seeds for now. Enter your code 3 FOR 2. The cart will most likely show the coupon applied, but a value of zero. If you view your cart it may only show one pack of seeds in there. Add a couple of more packs and you should see the coupon applied. Tinker with the amounts and add the seeds YOU WANT at this point and the code should remember and apply accordingly.
There's obviously an issue with the site but you can get it to work with the trick above... or at least I did.