It's been bugging me for a while but I managed to get it dealt with today.
It's all to do with international domains... but first I need to make the point that there are some products, like postage stamps, binders, edibles and others that are only sold on Zazzle's .com domain.
The problem was that the international domains feature of Nifty Slideshows was showing the USA-only products to people outside the USA. Because the link was adjusted to use the Zazzle domain associated with their country, if they clicked say, a postage stamp, they'd be taken to an error page on Zazzle - postage stamps not being available on their country's domain.
Well, I did battle, I lost, I re-grouped, got smacked about some more, had a cup of tea, wrestled the beast again, saw an opening in the armour and finally scored! (Not to mix too many metaphors!)
The end result, follow this link and if you're in the USA, you'll see a postage stamp near the beginning and some cake pops and other edibles towards the end. If you're not in the USA, you'll not see those.
Yaay!
I've still got to make the same changes to the code snippets generated for blogs / websites.