Not to go around in circles but if the fundraiser is taking a % its the same thing
It may appear to be the same but in essence vastly different.
The organization would much rather the cash/cheque (yeshivos, gas and almost anybody that can - tacks on the 3% fee) in contrast to the fundraiser, in which, organization would rather it be given to the fundraiser (creates relationship btwn donor and organization, makes it worthwhile for the fundraiser to be employed etc.).
And final difference is how the organization chooses to compensate/incentivize its employees or even allocate their expenses will not have any bearing on the contribution as opposed to a charge that is specific to your contribution and cannot
I agree that if you can only pay on CC - and cannot pay cash/cheque for whatever reason - since organization would gladly take a guaranteed donation even if it is on CC and eat the 3%, therefore you would be able to claim the FULL contribution for maaser.
End of the day, if you have a choice of contributing cash/cheque/cc and contribute CC, I cannot see how you would receive credit/maaser for the full contribution.