I agree that time is of the essence. Once dirt and water enter the crack it's hard to get the resin in there. When I get hit by a rock now I drive straight to the auto glass, also under $40.00 around here. The shop I go to will credit the repair cost towards a windshield if it doesn't work, but I've had 4 done and all of them worked.

If the radial cracks are only in the outer layer, and they often are, you can ask them to drill a small hole into the plastic layer at the crack tip. Not all of them want to do it because it's tricky, but if the tip is eliminated you're pretty much guaranteed it won't spread