Going to change the subject completely for a minute.
Alcohol.
Cooper ordered one bourbon and 7 Up at the beginning of the flight to Seattle, spilled it, and did not order another.
A drink menu from around 1959 has circulated listing the various bourbons available en route. However, by 1970, as near as I can tell, NWA had ceased listing brands and had resorted to listing styles such as Canadian, rye, bourbon, etc.
So, that said, in the 1970s, one of the most popular drink orders was a "7&7" which was Seagram;'s Crown 7 whisky and 7 Up soda. Catchy name, and that meant catchy marketing. Today, if you order a 7&7 you're probably in your 70s.
Also, today, Crown 7 is considered a "cheap booze". It's a dive bar whisky intended for bar flies and homeless people.
Bourbon was, and still is, a more sophisticated choice for drinkers. While there are varying tiers of bourbon - some expensive and some not - by and large, bourbon is a better whisky than Seagram's 7.
Why then, would Cooper choose a bourbon over the ubiquitous Seagram's 7? Was Crown 7 not available on board? Seems unlikely considering its popularity at the time. Perhaps Cooper chose bourbon to appear more sophisticated? To appear above his station in life?
If that's the case, then why mix it with a cheap soda like 7 Up? Whisky snobs will tell you that bourbon is sweet enough to be indulged in neat or over ice. They eschew using bourbon as a mixer in a high ball.
To me, this indicates that Cooper was trying to appear like a sophisticated, well-to-do man with high brow drink choices, but was still a blue collar or middle class guy who enjoyed his 7&7s and Schlitz beer. This was his one chance to show off, yet he failed by mixing a good liquor with a crummy soda.
Also, bourbon is made from corn mash. This makes it sweeter than rye whisky which is made from rye wheat, or scotch which is made from barley. Thus, mixing a sweet whisky with a sweet soda might indicate that Cooper had a bit of a sweet tooth.
Lastly, the fact that Cooper ordered just one drink and did not order any after he spilled his first, indicates that Cooper was not a heavy drinker. His ordering the drink was probably more for show than to actually get intoxicated.
So, in the end, Cooper's order of a bourbon and 7 Up demonstrates an inexperienced drinker trying to appear sophisticated and upper class while revealing his true blue collar or lower middle class social standing. It might also demonstrate Cooper's sweet tooth.
This is all elaborate conjecture based on a very small detail of the case, but I got tired of hearing people argue about people who obviously aren't DB Cooper.