[chrisseaton]

'Round up charge'? Do you mean 'round up change'? It's the money it would take increase your credit card charge up to an integral number of dollars.

But I don't know why that's a suitable figure to donate to charity, over any other.

[harrisonmgordon]

What makes change special is how small it seems on a day-to-day basis. We toyed with $x/mo vs. change, and feedback showed people were more likely to give if you describe it as change. I believe a part of that is knowing that what you give is tied a bit to what you spend.