Your original sentence is fine if it is for an email or letter.
It might sound a bit too formal for a spoken sentence though.
As for 약속드리겠습니다 versus 약속해 드리겠습니다, I would prefer 약속드리겠습니다. They both mean the same thing, but 약속드리겠습니다 (or 약속드립니다) seems much more common. -을/를 약속드리다 is used idiomatically in formal contexts.
There was a comment about using 귀사 or 귀 여행사 instead of 연세 여행사 but these 귀- expressions are really old-fashioned and formal. I think using the name of the company as you did is more in line with recent trends.