There's a need to look at the offer specifics and in some cases to query customer support. Some of the monthly offers pay downstream monthly added miles, and others do not. Some of the ones that don't pay follow-up months tell you how long you have to continue to donate for that first 500 to be OK (for example, if you stop in the first 2 months your miles will be rescinded); others do not.

One offer that popped up for me this week didn't give the minimum dollars or say how long you needed to stay or whether you'd get follow-up miles. I wrote, and this one for example needed 6 months of donations and did not have follow-up miles. That rated a pass.

An important fine print detail - you can't count on follow-up miles if they don't promise them.

FTF