I was a supervisor for 2 years at UPS. They are physical jobs. If FedEx is where you decide to go work it is going to be a physical job. Those jobs are fast paced too and you will get sore. And it really will last around 3 weeks if you're not lifting the packages correctly. The 3rd week isn't even soreness anymore, it's more pain. Just lift with your legs and you'll be alright because you don't want your back being what is sore for those few weeks.
As far as getting a job at UPS here soon and just being hired as a seasonal worker..... You might only be there for like a month through peak season(december), maybe not even that long. And the job they give you will not be very physical at all because you won't be a real employee and they can't risk you getting hurt. The job they give you will be extremely boring though. If you want to try to get on there as an actual employee tell them you plan on working there for at least 2 years even if you only plan on being there a few months. UPS doesn't pay much and you will have union initiation fees and monthly dues, but if you plan on being there for a longer period of time you can make very good money and they have the best benefits that you don't have to pay a dime for. One of my employees was only there about 9 months and got a job driving the brown trucks which starts out around $40K/year and tops out around $75-$80K/year 5 years later.