One product only be set to Flat Rate shipping?

I use the USPS shipping Calc. but it doesn't allow for the flat rate envelopes. I have one item that we want to ship in those flat rate envelopes, so how can I handle it differently? Can I have a flat rate fee added for that item and the rest use the calc?
For example, one person bought the book and it was $8.00 shipping, yet its only $4.60 with the flat rate envelope.
I have considered making it free shipping and add the $4.60 to the items price, but not sure how the higher price would go over even IF postage is already added. Is that my only option?

Thanks ~

Maybe I should be asking if there is a way to add USPS Priority Flat Rate Envelope as a shipping option.
I went ahead and changed the price on the site to reflect the price of shipping is included, but it says "Free Shipping" all over it and that's not true either.
I put in red at the top of the product page "Prioirty Flat Rate Envelope shipping included" ... of course it now shows the higher price to include it and not sure the author is going to be happy about that one.

Anybody???:lookaroun

What is the total weight?

Flat Rate Envelope has a limit to it for the $4.60 of 4lbs ... what is your Tare weight?

Well, that is where our POs differ, our's has always said that you could ship gold bars in a Priority Flat Rate ... as many as could fit and it would still go for the $4.60...while other's have said their POs place a limit.
Either way, the book in this situation ships at 2 lbs.