Translation:I should type that letter for my boss.
PIV says (under estro):
Rim. La diferenco inter estr k ĉef estas tia, ke la unua insistas pri la ideo de efektiva ordonpovo, dum la dua elmontras precipe la altrangecon inter pluraj similaj.
i.e. something like
The difference between estr and ĉef is that estr implies the idea of effective commanding power, while ĉef shows chiefly the higher rank among several similar things.
"This" means something closer to you than "that" (e.g. "This car" is next to you but "that car" is across the street). "Tio" and "tiu" are both "that". You add "cxi" if they are close (i.e. "this" instead of "that"). "Cxi tiun leteron" would be "this letter". You use "tiu" here because it is specifically the letter you are talking about. "Tiu" means "that" when there is a specific thing you are talking about (letero here). "Tio" is used for "that" when you aren't specifying what "that" thing is.