That's a completely different question. Getting 2% cashback may or may not entitle you to a full $100 maaser deduction. The discussion here is about the commissions and operating fees "the organization" pays. The donor still donated a full $100 which is entirely considered tzedakah, despite that a portion of it goes towards commissions etc.
Now let's say you aren't getting anything back in cashback but since you're using a CC, inevitably the organization will have to pay the processing fees. I don't see why that is different than the commissions and administrative costs they have. At the end of the day it costs you a full $100 out of your pocket which is entirely tzedakah.
In some instances, people are motivated to give mitoch shelo lishma or only because of how convenient it is. This costs the org money, whether fees for taking CC (and the donor likes the points) or costs of advertising, or costs of sending a self-addressed stamped envelope, or the costs of running a Chinese auction (even if the prizes are donated!). Is the consumer being asked to pay for these? Sometimes yes ("Please add 1.5% for CC fees"), mostly no. Does that extra cost to the org come at the expense of the donator (and his maaser)? Why should it?
I remember hearing a story about Rav Chaim Volozhin's meshulach: (Found here -
https://torah.org/torah-portion/ravfrand-5772-vayakhel/?printversion=1)
The “Volozhiner Yeshiva” had several such charity collectors who went around from city to city and from village to village in various locations in Eastern Europe. One of them came to the head of the Yeshiva, Rav Chaim, and told him that there were flaws in the system. In those days, the way a meshulach travelled around was with a hired wagon driver, who provided the rough equivalent of a car service. The meshulach complained that he was wasting his time and the Yeshiva’s time with the inefficiency of the setup. First, it was not easy to find wagon drivers and then the wagon drivers had multiple stops with many passengers. The system lacked efficiency. He argued that if he were given his own horse and cart, he could save significant time and be much more effective for the Yeshiva. He further said that when he came into donors homes shabbily dressed, he did not make a good impression. He suggested that if the Yeshiva could afford to provide him with a proper suit and coat, as a type of uniform to indicate he was representing a high class institution, the results would be more impressive.
Rav Chaim Volozhin was sympathetic to the meshulach’s arguments. He understood that “to make money you need to spend money” and he authorized the purchase of a horse and cart as well as a dignified set of clothing for the Yeshiva’s designated fundraiser.
Rav Chaim had the practice of reviewing receipts from each city where he would send fund-raisers to collect. There was one Jew in a certain city who always used to give generously to the Yeshiva. He was not a wealthy man, but rather a simple villager who nonetheless gave a sizable donation each year, way beyond his means. Rav Chaim noticed that the next time this particular meshulach went to this man’s village, the man did not give anything at all to the Yeshiva. Rav Chaim inquired as to what happened. The meshulach told him “I don’t know. I came into him like always and I expected a warm reception and a sizable donation as usual, but this time he gave a cold shoulder and no donation. I don’t know why!”
Rav Chaim was perplexed. He told his meshulach, “Let us both go together to see this fellow and ask him again for a donation.” They both went and knocked on his door. The fellow was blown away that the greatest Rabbi of the generation, the famous disciple of the Gaon of Vilna, was standing before him! He invited them in and after the initial niceties and refreshments, Rav Chaim got to the purpose of his visit. “What happened? All these years you gave us such a nice donation, and this year you cut us off completely?”
The simple Jew responded as follows. Every year when I sacrificed and gave my contribution I thought to myself: “I am giving to the Volozhiner Yeshiva — the pre-eminent Torah institution in the entire region — so young men can sit and learn Torah and develop into great Jewish leaders.” But when I saw your well dressed meshulach come with his own cart and his own horse I asked myself “Is this what I’m giving my money for — for a horse and cart and finery? I wanted my money to go for the study of Torah not for transportation costs and a uniform!”
Rav Chaim Volozhin told him, I want to explain the matter to you by interpreting a pasuk in Chumash. After the Torah already told us “And G-d filled Bezalel with the ‘Spirit of Elokim'”, what does the Torah mean to tell us when it adds the fact that he is one knows “lachashov machashavos” [literally to think thoughts] to do with the gold, the silver and the copper?
Rav Chaim explained that Bezalel was given a special form of Ruach HaKodesh [Divine Spirit]. There was gold used for the items in the Mishkan that were at the pinnacle of holiness – the Aron, the Menorah, the Shulchan, and the Mizbeyach (of incense). There was also money used in the Mishkan for things that were not as holy, for example, the boards and the curtains. Bezalel had the Divine Spirit to know that when a person gave money strictly for the sake of heaven in a highly spiritual intent, that was money used for the holiest of the keylim. When a person gave money with ulterior motives, then Bezalel knew to earmark that money for the boards, the curtains, or the less “holy” utensils in the Mishkan.
This is the interpretation of the pasuk: Bezalel looked into the precious metals that were given and he knew why they were given and therefore what to do with them. The same is true with donations to our Yeshiva. There are people who give money strictly so that the young men can learn Torah; they are not interested in any of the amenities or the bells and whistles. However, some people cannot focus in on the main purpose of a Yeshiva. If they see a physical dump or they see a representative of the Yeshiva who is poorly dressed without adequate means of transportation, they will be turned off by that. It is for the latter category of contributors that I had to dress up my fund-raiser and give him his own horse and wagon. Rest assured, I know the holy intent with which your contribution is given. Your money goes only to support the Torah learning of our students. It is the money of the other people who focus on material matters that is used to purchase the horse and carriage and wardrobe of my meshulach.