It's not that hard to add to a real website, but what percentage of transactions are conducted in it?

I mean if some big retailer like Amazon or BestBuy or Walmart to accept it, but only 0.02% of their daily transactions involved bitcoin would that be a success? It's a lot of money in absolute numbers, but it's minuscule as a percentage.

If it were 5% that would certainly be something to look at.