Facebook credits with much higher payout on Mobile Phone option
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I'm developing a Facebook app and I sell a digital product for 10 credits. In the credit card payment, the price is 1$, but in the mobile phone option the price is 2.5$. This is true for another credits amount. Let's say that I sell a product for 20 credits, the credit card price is 2$ but the mobile phone price is 4.7$.

Is this usual? It seems to makes no sense at all, as the price is more than 100% higher when paying by mobile phone!

Why is this happening?

Many thanks

P.S.: I'm portuguese, client from Vodafone.