You get what you pay for...

Czech is very cheap, their economy is much poorer than Germany or the UK, so their wares are also much cheaper...
But be careful, the Czech stuff is usually very closely scrutinised, when it comes into the country, at least in Germany, as Czech and Poland are renowned for selling cheap, poor quality copies of original products (Puma, Adidas etc. are constantly organising razzias against the market stalls near the western borders of the east European states).
I'm not saying that the Czech site is selling dodgy DVDs, but you should be aware that the DVDs could be destroyed by customs, if they are not originals and you will still have to pay the import duty - and try and get your money back off the site... I would definitely check the reputation of the Czech site (and the German one, for that matter), before ordering.
You can try and get a discount from the German site - be foreful, but ALWAYS be polite, otherwise the price is likely to go up!
