This can be any or either of the following osCommerce issues:
1) "shipping tax" bug:
Ensure there's no tax calculated over the shipping method, in checkout osCommerce will calculate the tax twice for some reason. You can find this setting under: Admin > Modules > Shipping (Verzending) > Select the shipping method > Tax Class
2) Currency calculation
Although the currency might be different in the shop, perhaps selected by the customer, the payment method might use a different currency (such as iDeal only supports transactions in EUR). osCommerce will recalculate the price according to the osCommerce currency rate or if the currency has not been configured an error will be displayed: IC_ERR_B: IC_Amount must be numeric