Actually, as long as you make enough money, you can claim the entire donation as a tax write off, not a percentage. First, the total donations for the year (which would be more than just the card) would have to exceed your standard deduction. The standard deduction is something like $4550 for a single person or $7600 for a married couple or widow/widower (the numbers may be incorrect, but they are relatively close). Then, the charitable donation deduction cannot be greater than a certain percentage of your income, Im not sure of the number. So, like I said, if you make enough money, you can claim the entire card as a write off.
And for EdipisReks, if your friend moved up in payscale to the highest tax bracket, its impossible for him to "earn less". I dont know the cutoffs for the 5 brackets, or the rates offhand. For example, if he was making $120k a year, and the rate in the 4th bracket (and we'll say that amount is the cutoff) was 30%, then moves up to $145k a year and into the 5th bracket, ONLY the money ($25k worth) above the 4th bracket cutoff is taxed at the higher rate. So the first (I dont know the exact numbers or rates, this is just an example) $15k will be at the lowest rate of 10% or so. Then the money beyond that up until $40k will be at the next bracket of 17% or so, with the first $15k only being taxed at 10%. Then the money above $40k up to $90k will be at the third bracket of 24%, and the money between $15k-$40k at 17%, $15k and under at 10%. You get the point, right? Any tax breaks for the lower brackets trickle up for the higher tax brackets. Thats why GWBush's tax cut looked proportionate, until you realize the cuts for the 4 lowest brackets ALSO went to the highest bracket, before they even got their tax cut (which was still the largest percentage).