Business Writing (Grammar)

So I've recently received an assignment back from my Business Writing class.

The assignment was to write a letter to a client addressing the claim of their unsatisfactory product. In the letter, I wanted to apologize to the customer for the inconvenience that the company has caused them. To make up for it, I wanted to offer the customer a 20% coupon off their next purchase. Thus, I have the following sentence in my letter, "For your inconvenience, we have enclosed a coupon for 20% off your next purchase."

The professor made the following comment, "I know this is an error, but it is a good example of how you have to closely scrutinize your document before releasing it." However, it is not clear what the error is. (Talking about the word "however", she says that the word "however" should never start a sentence so I've lost an entire letter grade on this assignment because of that as well).

I suppose that the sentence could be misinterpreted to mean, "To inconvenience you more, we have enclosed a coupon for 20% off your next purchase." However, what about the very common phrase, "We apologize for the inconvenience."? That could be arranged to, "For the inconvenience, we apologize" which could be misinterpreted the same way to read, "To inconvenience you more, we apologize."

Any help would be appreciated. Thanks!