The first thing you need to ask yourself is, "Am I sure the only damage to the lens is to the mounting flange?"

The second thing you need to ask yourself is, "Considering the cost of a new EF 75-300mm f/4-5.6 is only around $200 and a used one can be found for half that, why would I consider sending the lens in for a repair that will likely cost near that just in labor cost and a replacement flange ring?"

Any force that managed to bend the bayonet lug more than likely managed to knock the alignment of the various optical elements inside the lens out of whack as well. Finding any shop that does quality lens alignment once a lens has left the factory is difficult at best. Finding one that will do it for less that the cost of an EF 75-300mm f/4-5.6 is pretty much impossible (as discussed in both this blog post and comments and this one from Roger Cicala at lensrentals.com).

If you can repair the flange mount ring enough to get it to mount (without forcing it - force it and you could damage the camera's mount ring) you could then test the lens to see if the optical performance has degraded compared to before you dropped it. Otherwise you're probably better of just writing it off and replacing it. If you're shooting with an APS-C camera you might consider the EF-S 55-250 rather than another copy of the 75-300 that many consider the worst lens in Canon's current lineup. For a minimally higher price you get better optical performance and Image Stabilization. You give up the 250-300mm focal length range but you gain the 55-70mm focal lengths.