The only reason I would work at UPS while instructing is for the benifits and income, not to mention you do keep in shape. The letters would be a bonus though. UPS likes letters from managers and if you can get on as a Next Day Air Delivery Driver, or any other part time delivery driver position, they dig that also.
I worked at UPS for 5 years while going through college /working on my pilot ratings and enjoyed it. It all depends on your location and managers though. I quit UPS after college to instruct and flew over 1100 hours in one year, so I never had time stay working at UPS. If you’re looking for a supplemental income and benefits while instructing, it would be great. I also got a chance to go take a tour of the facility in Louisville and fly the 767 sim which was pretty cool. I just got my commercial cert and really had no clue what I was doing so I didn't get a chance to impress anyone, but it was cool just the same.
I kept in contact with a one UPS pilot and a couple UPS center managers thinking I might call upon them once I got some turbine PIC time, but I recently got hired by another airline which I plan on going to and staying.
I know of someone that went over as a management pilot and he quickly found out how divided management is with the union. From the ground side of it, it's pretty hostile. I imagine the air side is the same. I'm sure management is trying to scheme up some way to get pilot salaries down to industry average. I'm not sure I would want to be there in the future when they start hitting the pilots up for pay cuts or outsourcing. Anymore, in this industry, it doesn't seem like the top paying jobs stay that away, although China is opening up. Just my take.
To sum it up, I would concentrate on your flying. If you find yourself with some open time you can schedule students around, needing some medical benefits or extra cash, and don't mind working hard, UPS would be a great option. I don't think I would work there just for a letter of recommendation. I would rather hang out at a bar where some UPS pilots hang out at to get that.
Good luck