Very physical - at my current center (of 5 years) I've seen 4 or 5 military reserve employees quit due to physical stress (they actually walked out mid sort). When working a night sort (local sort), summer temperatures in the trailers average over 140 degrees. During the winter, preload sorts (morning sorts) can be single digit temperatures depending on where you live - and you WILL work in it.
Though I listed it as a pro, seniority based job advancement can be terrible in specific cases.
You can only transfer as a part-timer by means of education, death in the family, or mortal health/sickness. Transferring is impossible if there aren't any openings.