Neither is correct, since you sent something to somewhere, not at somewhere. To indicates direction.
Otherwise while there is a recommended order to adverbs or adverbial phrases, both are fine and natural English. But if you want to follow the recommended order, it is:
- Manner
- Place
- Frequency
- Time
- Purpose
So, in your example:
I sent a book by bus (manner) to the head office (place).
Other examples:
I have to run quickly (manner) down the street (place) each morning (frequency) after breakfast (time) in order to catch my bus to school (purpose).
She buys dinner from the shop (place) every evening (frequency) for her children (purpose).
However, I think this "order" is as often broken as not. For example, when using verbs that imply movement it can be better to put the direction or location first, to avoid confusion:
I sent a book to the head office (place) by bus (manner).
Or you just order it by what you think is important:
She buys dinner for her children (purpose) every evening (frequency) from the shop (place).