1. Is writing a thank you letter a must after an interview? especially when you've been offered the job? Or does it depend on the position of the job?
2. I went to an interview at company XYZ and they told me that they will make a decision on who to hire by November X. However, they also told me that if I do receive an offer at a different company, I should let them know. Should I write them an email to let them know? A formal thank you type of email? Or was that said out of courtesy?
Hey there,
I worked for our campus employment and career centre for 2 semesters and i think i can give some advice with your queries there.
1. It's good to send a thank you letter because it shows that you're really interested in the job and that you value your potential employer's time. If you're not really into the job, say... it's something that you're not really interested in and you would like to completely cut ties with the company you've applied for, then i don't think it's necessary though. Doesn't hurt to send thank you letters though... even if it's short, as it shows that you're professional and that you're grateful for the time they've spent interviewing you.

It leaves a good impression.

2. If i were in your shoes, i would send them a thank you e-mail then follow it up with the previous discussion about what they told you to do.
Example:
Dear XYZ company,
Thank you for interviewing you. Your company has been.. *insert words of flattery about the company and the people who interviewed you and what you found admirable in the said company*.
As what was previously discussed in the interview, i have not heard from your company, but i have been offered another position by company ABC recently... *continue on telling them what has happened*
Once again, thank you for your time and i do hope you find a suitable candidate for the said position *if you want to work for the company XYZ, you should let them know about it in this paragraph as it states your intention for the letter*
Regards,
You!Hope it helps.

Good luck there!

i'm a social worker.
i'm totally rolling in the dough.
CHA CHING!!!!!!
..................................not.
hahaha sigh.. for the good of the people!
Good job!

Thank you to our dear unsung heroes, such as yourself!

Power to people in community services!