Assuming that a donor itemizes deductions and doesn’t use the standard deduction, the donor is entitled to deduct an amount donated to or on behalf of a charity.  Here, for example, if a donor gives a charity $100, the donor is entitled to deduct the full $100.  It doesn’t make any difference how much the charity pays to receive and process the contribution.  Those fees, including the cost of electricity to maintain the internet connection if the gift came online, the credit card company charge, the low-cost tchotchke you give back to the donor, the compensation of the fundraiser who asked for the contribution and the bookkeeper who recorded it, are all costs of the charity, to which the donor is contributing.  The payment of the credit card fee would be a legitimate expense for the charity, but, like all of the other administrative costs of the charity, would not reduce the amount of the donor’s contribution that is deductible by the donor.  

In some cases, an online platform may ask a donor if the donor wants to pay an additional couple of dollars to cover the platform fee for the charity.  In that case, if the donor agrees, the additional amount would be a payment on behalf of the charity, and would also be deductible.

You should be aware that the IRS instructions for the Form 990 annual tax information return make clear that the charity is required to report the “gross amounts” received by the charity as contributions on the statement of revenue, and to account for the expenses separately on the statement of functional expenses.