okay, for the record, i did go back (yesterday) and i did pay for the crank arm. he only charged me for the left one, so it came to about $50. before he rang me up, the salesguy goes "are you sure steve didn't charge you on saturday?" i picked up my friend's old derailleurs that i had left on the counter, too. problem solved
now, both of my parents are small business owners, and often their employees will make mistakes and undercharge customers. very rarely does a customer ever come back in and admit that they were undercharged and, if they do, my parents generally thank them for being honest and admit their employee's mistake. they never demand that the customer then cough up the dough.
oh, well, everyone runs their businesses differently, and they certainly had a right to the $$

and the "beast" part... that was the sales guy's word, not mine. i found it ironic because i am not beast-like in the least... kinda wimpy, actually i think the crank and derailleur were probably damaged in transit on the back of my car. *sigh* poor bike....