New pedal fell off maybe 50 miles after installation. Should I replace the entire crank, just the arm, or repair the thread? Repair newbie here just getting back into mountain biking after a long absence. 

I recently bought a Cannondale Catalyst 4 from REI. It's a fine introductory bike for what I'm looking for, but I hated how cheap the pedals felt. So I replaced the stock pedals with these:

Unfortunately, maybe 30, 40 miles after the installation, the left pedal fell off today. When I tried to put it back it seems like the threads on the arm are completely stripped...

I can only assume I messed something up with the install. Probably didn't thread it properly or tighten it fully. Oh well, learning experience. 

I'm just unsure what the best way is to replace it. I see these as options:


Repair the thread (can't do myself, as I don't have a workbench or the tools to do so).
Replace just the arm.
Replace the entire crank.
Return the bike to REI and upgrade to a better one, since I'm fairly confident I want to buy a full suspension bike with a dropper post and better everything. 


I appreciate any advice.

EDIT:

Thanks for the replies everyone. Local bike store is going to replace the crank for $15 + labor. Should be good to go from there. I'm going to be more careful with my maintenance from now on!
If you installed the pedals incorrectly, you likely damaged the other crank as well.  I'd buy a new set of pedals and have a competent repair shop heli-coil the cranks (both, if necessary) for you.  I believe this would be the cheapest fix (if you plan to keep the bike).

As for the return and other issues, I have no idea what REI's policies are and whether or not they would take back a damaged (and currently unusable) bike.  Perhaps they will refund your purchase price minus repair costs.  Again, heli-coil is likely the cheapest repair.  However, you need to talk to them to find out what options they are offering you.

Based on this incident, I'd recommend you have a shop do any repairs you need until you get some experience wrenching under your belt (from a class, competent friend, internet, etc).  That is, whatever you decide to do (new cranks, heli-coil, etc) I recommend you do it with some sort of supervision or instruction to save yourself additional hassle.