Which Amazon do you chose?
Which Amazon you chose is determined mainly by where you live.
US Amazon amazon.com is based in the US and so payment is made in US dollars. Videos available from here are in NTSC format and are for use in the U.S. and Canada.

UK Amazon amazon.co.uk is based in the UK and payment is made in pound sterling. Videos from here are in PAL format, which is used in the UK, Europe and Australia.

Of course, this is only a guide and because some items might not be available on both sites, you can make purchases from whichever Amazon you want to.