How to calculate international credit card IOF?

public static bool Mod10Check(string creditCardNumber)
{
     //// check whether input string is null or empty
     if (string.IsNullOrEmpty(creditCardNumber))
     {
         return false;
     }

      //// 1.   Starting with the check digit double the value of every other digit 
      //// 2.   If doubling of a number results in a two digits number, add up
      ///   the digits to get a single digit number. This will results in eight single digit numbers                    
      //// 3. Get the sum of the digits
      int sumOfDigits = creditCardNumber.Where((e) => e >= '0' && e <= '9')
                .Reverse()
                .Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))
                .Sum((e) => e / 10 + e % 10);


       //// If the final sum is divisible by 10, then the credit card number
       //   is valid. If it is not divisible by 10, the number is invalid.            
       return sumOfDigits % 10 == 0;            
}

So I realized my answer is no longer correct so here is a correction I keep the other one for future reference.

As I realized (and I am from Portugal, we do not have IOF), the tax is currently 1.10% of the total amount.

There are two ways to calculate with aggravation or included in the total value of the purchase.

1) Worsening

 var valor = 100;
 var total = valor + (valor * 0.0638);

That is, 106.38.

2) No Aggravation

 var valor = 100;
 var total = valor - (valor * 0.0638);

That is, 100 being the amount payable without IOF is 93.62

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