There is no need to give in to Lodsys on the first letter you get. You should not panic. It is only a first letter. If you sign you are on the hook: you may not talk about th agreement, you can put driven to substantial costs if they "don't believe your sales reports". And that will continue even if the patent gets invalidated, which is not very unlikely.
Lodsys is extorting. You think the 0,575%, is not much, but it are the hidden conditions that affect your whole business and that may put you out of business.