There was an actual 99 cent charge on the statement. So hey, it is possible.
There are two fees that are associated with credit cards.
The first is a monthly access fee. The second is a transaction fee. The monthly access fee tends to be somewhat standard. The transaction fee varies from company to company. Big retailers like iTunes are able to make deals limiting this fee.
Last I heard, the breakdowns for iTunes are roughly the following:
$0.69 - goes to record label
$0.10 - goes to credit card companies
$0.05 - goes to internet company
$0.05 - operating overhead
$0.10 - profit
Now, those are likely averages and not a pure breakdown by song. So the credit card fee may actually be $2.00 but there are enough people bying 15 songs at a time or gift cards to make up for the people buying $0.99 songs with each purchase.
Back to the PSN, the minimum required purchase is probably to cover the two credit card fees. Sony isn't the only company to do this to protect themselves from a large number of small transactions. Especially if they envision future content to be rather inexpensive, this is a fairly good safeguard.
Retailers don't usually disclose the exact amount that the per transaction fee costs them. For good reason too. I wish I could find the story, but not too long ago there was a lawsuit involving an internet site. The person lost and was ordered to pay a fine. Because the judgement provided that the fine could be payed in checks and set up a fund to allow multiple deposits, the person asked all of their supporters to send checks for $0.01. I believe the court stepped in and stopped it, but if that had been allowed, the person collecting the debt would have lost an enormous amount of money on check cashing fees. If retailers like iTunes that don't force a minimum transaction fee gave out hard information on their transaction fees, then they would open themselves up to this type of attack.