And someone(?) translated that to:
"The letter was sent to the Tofugu office."
While I would not necessarily call this an incorrect translation, it is certainly not a literal one, either. Why not?
Because in the strictest sense, 「届ける」 does not mean "to send". It means "to deliver". So, at least my TL would be:
"A/The letter was delivered to the Tofugu office."
"By the post office", "by the delivery service", etc. is just left unsaid in this sentence (because that is not very important information).
The 「に」 in 「オフィスに」 is a destination marker.
If it were in deed "the office" that delivered the letter, it would be far more natural to use 「オフィスによって」.
I understand 100% where your question is coming from and it is a good question. In this case, however, the context tells us that the 「に」 marks the destination.