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.
I’ve been looking at print-at-home options. If we provide the artwork and templates people can order and print their own for about $1 each if they print 20 or so. They will have to be sprayed with a vinyl coating as well.
I’m not looking at making these available on an individual basis (better to offer large quantities to organizations who can then distribute themselves)...too much time and $$, so being able to order in smaller quantities from a printer or doing at home will be necessary.
I think $3-$5 is to much, we need to find a way to get the cost down to around a dollar.