This is simply a way for the Assistant United States Attorney (AUSA) to make his job a little easier. If you refuse, then the AUSA will need to take the matter to a grand jury and formally obtain an indictment. If you accept this offer, it saves him the trouble of doing so.
As to the offer, you should absolutely speak to an attorney about this matter. It certainly could be in your best interests to accept the pre-indictment offer. If you refuse, it is possible that the AUSA would seek an indictment on more serious charges and then extend a more harsh offer.
I hope you found my answer helpful. If so, please click on the green ACCEPT for my answer. This is necessary for me to be paid for my work and so that I can get credit for assisting you. Even if you are a subscription member, you will need to click ACCEPT. Your question will not close, and you will still have the opportunity to follow-up if needed. Leaving a bonus and positive feedback is not required, but doing so is certainly appreciated! Thank you and good luck!
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.