I don't think your right. Read up thread.
Lets say a fundraiser comes to your door and collects for an institution. He is taking a nice percentage. Maybe you should tell him thank you for telling me about the institution I will pay them myself as I want 100% to go to the tzdaka. Furthermore, if you don't do that and actually give it to him, do you have to call up to find out what percentage he gets and deduct that from what you supposedly gave to maaser?
There is an integral difference. In your example, you are giving the organization $100, of which
they are giving 20% to the collector (I just made up random numbers). By using a credit card, you are paying the credit card company $100, for which they are giving $97 to the organization. Just because you are being charged more by the credit card company than the actual donation amount, doesn't mean that you can write off your credit card bill as ma'aser.
If you wouldn't pay the full amount and would have to pay interest on the amount you donated, would you deduct that interest from maaser, being as that's what the donation cost you? Of course not, as at the end of the day, that has nothing to do with what tzedaka received.
Credit card processing fees essentially mean that you are being charged 3% more than is being paid.
A more accurate example would be if you had a messenger deliver your donation to the organization, and deducting the delivery charge you paid him from maaser. If you send a check in the mail, do you write off the cost of the stamp? I think that may be a better parallel to credit card donations, and the logic I would see in a Rav paskening that.