i believe they're giving you the heads up *in case* you do get charged duties by customs, who do not know the value of the item, and can only base any taxable amount on the declared value which will be written by FSH staff on the shipping label.
You need to just prove that the bag was already purchased in USA, that the original customs and duties have already been paid for, and this is simply a returning item from Europe. There are some forms to fill out and usually the home boutique fills this out on the declaration form for you when you send in your items for repair/spa, because they are acting on your behalf.
However in this situation, you are the importer and since no original paperwork was submitted, customs will basically assume that you didn't pay duties, unless proven otherwise. It shouldn't be that hard, but will require a lot of coordination, phone calls, and emails. Good luck.