I didn't notice that your offer Paypal as a purchase option. Could it be that the examples I looked at were by non-US filmmakers?
Your solution of automatically signing people up without them formally going through the process sounds like a good one. Is this the same if someone is buying a physical item?
Certainly, implementing tax and VAT for every customer is going to be a challenge. But, I think if you make it customizable enough for each filmmaker, then it can be done right. For US based sellers, following the way it's done in Paypal, where you can configure both a state and zipcode range, will prob cover the bases (although determining the exact zipcode range you need can be a challenge, as I learned here in LA). For sellers from other countries, you'd need to see how comparable services handle those details, but I'd guess it would be a matter of determining similar factors: country, state/territory, city, postal code. Obviously, it would be great if the software automatically figured all this out. But realistically, if you let each client configure the rules the way they need to be, that could be good enough... (although it might be nice to have some recommendations for major areas.)
The thing I'm unclear on is the digital end. Is tax ever charged for streaming & downloading? I took a look at Journeyman Pictures, after Nigel mentioned it, and I noticed that they charge VAT in Euros (and the one download I looked at cost even more than a typical DVD!).