Do you purchase something once on Playstation Network, or do you pay monthly?

I never purchased anything on the Playstaion Network. Today, I bought Mafia 2 with the Betrayal of Jimmy pack. I looked at the Downloadable content, and I can also purchase a few others for $2.99. The packs are Vegas Pack, Greaser Pack etc. Do I pay $2.99 once, or is it a monthly thing where it adds up to be a lot?