I agree...however, they may be hesitant to put anything in writing unless you know one of the SA's very well. I can't imagine they are too thrilled when people come in whining about bags they purchased outside of the boutique not being as described or possibly not being authentic. If you want a new authentic bag, purchase it there! That is why and how they get paid! They will give you a verbal estimate, but the final cost always depends on what the repairers quote them when it is eventually repaired. Since the figure is subject to change, they don't usually write it down. If you are lucky enough to find an SA who will do it, see if they can cite the creased bottom as the reason for repair.
I think this whole letter business is positively ridiculous. If Chanel won't provide these letters, they are asking people to spend additional money which they won't be refunded paying someone to do this...and ultimately how is that person anymore qualified to detail the damage? In the end it is still ridiculous because what's to stop the buyer from causing damage or furthering the damage just so they can bring it to a professional for a letter. IMO, this solves nothing and is a waste of time and money.