What's the extra charge for using iTunes?

Everytime I buy an app from the app store from my iPhone, I get charged the 'app' charge and then on my statement I could get charged anything between 6-8 euros described as a 'finance purchase charge'
What in earth is this?
Every app I buy is costing tons extra. Has anyone else experienced this?
I'm living in Ireland, and use AIB Visa card.
Thanks for your help.

Added (1). I should add that this is nothing to do with interest charged or the card being overdrawn.
For example. If I had a credit card that was zero on the 1st of the month why would there be a charge of 6-8 euros each time I use the app store through the month?

That's just the interest on your unpaid balance. Pay your credit card in full each month, or pay the interest.

It has absolutely nothing to do with iTunes.

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