8.99/19.99 = 0.44872.
So yes, relative to the list price you selected it is 55% off, but that list price is your own invention.
However, to the two points raised, I also own that Powerport 2 UK port, it is good, it suffers from a travel perspective from the problem that UK sockets are huge. So I rarely move it, I tend to keep within UK. I move with USA products and attach a plug adapter. Smaller in total volume and weight.
I have a number of people spending weeks traveling and that UK Powerport 2 will be moving around a fair bit to non-geeks to keep phones, iPads, iPod, etc charged.