You will have to fill out a customs declaration on a USPS form, which you obtain at your local post office, or online at
www.usps.com.
Be sure to check the post office site for restrictions on mailing to France, and make sure your item is described as an allowable item. For instance, many countries restrict "currency", so coins have to be described as "numismatic items", "tokens", etc.
Any customs charges will be the responsibility of your buyer, not you. It is
not advisable to lie about the value or call it a gift if it isn't.
If you choose to send it by International Priority Mail, either with or without using a flat rate envelope or box, you can actually print a combined address/customs/postage label at the website, and pay for it with a credit card. You can specify delivery confirmation, which will be an email, or pay extra for Registered delivery with a return receipt. Insurance is also available with or without registery.
BTW, priority mail rates are slightly cheaper for online purchases than they are for using stamps or paying postage at the post office, and if the package weighs over 13 ounces you can still deposit it in a collection box without having to stand in line and give it directly to a counter clerk.