"appalling" that
The point is that the OP was unaware that the charity donation would be treated differently to any other credit card purchase.
It's like sending a cheque second class and then getting billed for special delivery.
It's not an efficient way to do it. The cash advance interest (and with some banks - a flat charge too) is likely to be more than the merchant commission, and the charity and the OP will only be able to claim tax relief on the donation after charges, whereas with a normal purchase they'll both be able to claim tax relief before charges.
Example - you give 100 paid for as a normal purchase. The charity claims 28.20 off the tax man making total they get 128.20. They pay a 2% commission on the 100 meaning they end with 126.20.
The donor pays higher rate tax and so claims tax relief on the 100 donation, of 23.08.
So the charity gets 126.20 for a cost to the donor of 76.92, ie 164% of the cost to the donor.
Compare with paying 100 as a cash advance. Charity gets 128.20 after gift aid, donor gets HRT relief as before taking his cost down to
76.92, but also is charged 4 interest/charges by his bank. So the cost to him is 80.62 and the benefit to the charity is 128.20.So the charity benefits by 159% of the cost to the donor.
In addition, if you add in things like cashback, which will apply to purchases but not cash advances, giving to charity via a cash advance becomes even more inefficient.