I know nothing about UPS stock, but I am a retired federal employee. I am wildly in favor of doing whatever possible to increase the size of one's TSP.
From 2003-2006, my TSP did extremely well in C, S, and I funds.
During the 2008-2009 economic collapse, it was nice to know that my TSP was invested 100% in the G Fund (to which I shifted it in late 2007, in preparation for retirement).
Now that I have retired, I mailed off the forms to start equal monthly withdrawals and I am looking forward to this income stream.
"You can never cross the ocean until you have the courage to lose sight of the shore." - - - C. Columbus