At least with Amazon, all you need is: http://www.amazon.com/gp/product/B003O6G5TW

Which isn't too bad...

Or even: http://amazon.com/dp/B003O6G5TW

or: http://amzn.com/B003O6G5TW

Luckily, everything from /ref= on is optional, and the alphanumeric code that comes after product is an actual product code. Replace it with the 9-digit ISBN for a book, and you'll get the book, for example.

That's intentional. They'd rather not give out a bunch of Google Accounts usernames (which would also be a lot of Gmail users).