Welcome to Coin Community, Marie.
If these coins have not seen circulation, every single one of them is worth at least $700, some much more than $700, and undoubtedly some are worth significantly (add a zero, at least) more than that. The very most common Liberty Head and Saint Gaudens Double Eagles, 1904 and 1928 respectively, are worth $700 in the lowest-quality Uncirculated condition. The price goes up from there.
You mentioned an 1858 coin. They were minted at three different mints (look for a mint mark below the eagle on the reverse, either none (Philadelphia), O (New Orleans), or S (San Francisco). There are fewer than 40 known Uncirculated examples from Philadelphia, about 25 from San Francisco and only 4 from New Orleans. If this coin is Uncirculated and undamaged, you can expect a substantial five-figure return from it alone.
So, upon what value is the French government going to place that 7% tax on? The local-sale value, at $700 per coin as you mentioned? If so, it is absolutely worth your while to pay it and get them to the U.S. where the ones you wish to liquidate can be disposed of in a venue where they will generate the best possible price. Here in the U.S., that will very likely be Heritage Auctions, who are the largest rare-coin auctioneer in the country.
First things first. For now, stop, think and do a little learning. Go to the Heritage Coins website:
http://coins.ha.com/You can register for free, and get access to their database of over one million completed auctions. It's the single best resource on the Internet for determining a coin's true market value.
Then, learn a little about how to grade coins. You don't need to become an expert today, but it will help you immensely to have an overview of coin grading standards. I can recommend two places. First, go here for a quick overview of grading standards:
http://www.acoin.com/grading.htmSecond, optionally, go here for a much longer course, which will set you up with as much information about grading as you can possibly use before actually holding coins in your hands to grade:
http://www.coingrading.com/intro1.htmlThird, wander over to PCGS for an overview of the Third-Party Grading (
TPG) process - there are only four or five reputable
TPG's, dozens of less-than-reputable ones, and I highly recommend you invest in having these coins graded. Here's what the
TPG game is all about:
http://www.pcgs.com/aboutus.chtmlAbove all, be aware that you are dealing with a process which, done wrong, could possibly mean the loss of value equivalent to the price of a small house. Do not feel any sense of haste - these coins are gaining in value every day.
As a preliminary step, you can compile a list of dates and mintmarks. Here's all you need to know about both types of Double Eagles you're concerned with:
http://www.coincommunity.com/coin_h...ad_type1.asphttp://www.coincommunity.com/coin_h...ad_type2.asphttp://www.coincommunity.com/coin_h...ad_type3.asphttp://www.coincommunity.com/coin_h..._gaudens.aspAccording to your wishes, you could either provide that list here in the forum for public comment on values, or privately contact either the Forum owners, Susanlynn9 and Bobby131313, or myself or one of the other moderators, and we'll be happy to help you work through what needs to be done.