Write-offs aren't magic. There's no way to write off a donation in a way that will reduce your tax burden to lower than it would have been without the donation.
If you donate $100 to charity and write it off, it reduces your income by $100. If you pay 30% in taxes, it reduces your tax burden by 30 bucks, so you're still losing $70 versus not mak8ng the donation.
The difference with these round-up measures is that the donation is only given if the customer pays for it, so it breaks even after the cost of implementation/overhead of the system.