It doesn't take long to cut through 50x50 3mm box section by hand with a hacksaw...just make the minimum number of cuts to get it in the car and do the rest at home.
The company I got my steel from cut it to length for me for no extra charge, but evidently they didn't know how to use a tape measure as some of the lengths were wrong! Luckily they were too long so I sorted it out. Not complaining though as they gave me some freebies to practice welding on which were very useful :)