'Life in the fast lane' is living energetically in some way: possibly a very busy daily schedule, or a lot of wine, women and song in your night life. The 'fast lane' is the lane of a multi-lane highway where the cars travel (or are supposed to travel) at higher speeds. This lane is at the opposite side from the 'truck lane' or 'slow lane'; in Japan, the fast lane is the rightmost one. Your customer's enquiry may or may not have been facetious (depending on your occupation or reputation), but basically, she was just saying, 'how are you doing?' / 'how's it going?'
I have no idea about 'screwing in nickels', but I think you should have made the customer cough up one, unless liberality has increased since my day. One cent, sure-- but 4 cents for free?!