The problem is anything printed at home is going to wash away the fist time it rains unless after printing one apply a vinyl cover to them.
It comes down to scale and cost... if you have people ordering very small quantities, something like CafePress (and that site is used just as an example, I have no experience or stake with them), you're looking at $2.99 base cost, say $0.01 "store markup", for $3.00 per sticker, plus $0.75 shipping for the first and $0.15 for each thereafter. The other advantage to something like CafePress is that the same designs could be placed on other products as well (t-shirts, mugs, buttons, etc.). The "store" would make $0.01 per sticker, plus potentially some volume bonuses, that would be donated to a specified cause.
For larger runs (10 or more), sending the artwork to someplace like makestickers.com (using Google's #1 hit for "custom bumper sticker") makes more sense. It's $4.95 for one sticker; but $2.70/each for 10 stickers, $0.90/each for 100 -- all shipping included. This is a much better option for someone who wants to get a whole bunch to distribute.
I don't think print-at-home is a very good option. By the time you factor in the cost for ink, stock, and vinyl covers, I doubt you'll save much (if anything) over professional printing.