That minimum charge is obviously not the same with every card merchant or country, which (I suppose) is why it never came up before. Of course, you don't usually expect to get e-mails from people saying, "I wanted to buy your book, but it was too cheap! Raise your prices, dude!"
One of the things that still needs further development is the micropay process. Most of the transaction fees and minimums we have today were set quite a while ago, and the advancements to the web, transactions and general efficiency, not to mention the fairly recent introduction of electronic-only products of potentially small stature, merit an adjustment to the fee system.
Unfortunately, the financial industries are almost as tightly entrenched in "the way we do it" as the publishing industry... one of the reasons we just witnessed the financial meltdown... and expecting them to make such a change will only come at considerable pain and expense... in short, we may eventually get it, but we probably won't like the hoops we'll be forced to jump through to get it...
Hmm... slightly
, methinks...