I worked in Louis Vuitton for four years as PR officer. I can verify a few things for you.
1. the letter is authentic. Baker & Mckenzie have been their appointed law firm. LV has a legal department in the office compose of an in house lawyer, investigators. They also work with outside Private Investigator to dig up cases of legal infringement. They are also very close with the Customs department in HK and mainland China. That's how your product was being photographed.
2. The letter is more to make sure you take away the products which they feel is infringing the "monogram" pattern. rather than to take your 25K fine. As many people have already told you, they don't need that money.
3. And from my experience, they send out so many letters to other people selling similar product like what you are selling, is to "clean the street" because the LVMH big boss Bernard Arnault is always visiting Hong Kong in early March. And this year the business is expected to be bad, the legal department will make sure the "street is clean" so that big boss won't be more upset...apart from bad business.
4. I"m not a lawyer, so this is just a suggestion from an experienced person who know how they work==>Bring the letter to the law firm. Tell them the truth, that you were not intentional about the infringement, and you do not have the money to pay them back. Ask if there is room for negotiation. As long as you show sincerity by saying you WILL stop selling the products, they should be fine. But you will have to sign the letter.
5. LAST RESORT, warning, this is risky but from my understanding, they will back out....if the law firm insist to sue you and will not back out after you've done everything on point 4, go to apple daily and oriental daily and next magazine. I will give you name of the journalists and make it a big scandal. As i said, they need positive things to show to French big boss, not negative. Someone will calm you down if you go to the press.
I truly hope this will help. And yes, you are being unlucky because it was never easy for them to successfully win any infringement in their monogram pattern. But people who gone into serious law suit with them are big corporation. Such as Sony music....they had a MTV of Britney Spears showing a car with monogram pattern. few years ago and the case was only closed after 2 years of court in the states. And Sony music loose but only paying a fine of US40,000 to LV and Sony will stop using that MTV. Imagine how much legal fee was involved in such case.