Today I mailed an incredibly light book -- one of those very thin mmps from the 1970s where the type is minute and the book weighs nearly nothing.  When the postal worker weighed my book at the post office, she said that her computer would only give her the first class "flat" rate, rather than the first class "package" rate, because it was so light, even though it did not qualify as a "flat."  Is there some point at which first class "flat" and first class "package" have the same rate because the package is so light?  Or should I expect a postage due problem on his book?

Thanks!

UPDATE:  Apparently the book made it with no problem, because I got a credit for it today.  Thanks again for your replies.  :->

I'm not sure - but I have a small digital scale at home, and I simply weight the package, plug the numbers in at the postal website along with the zip codes, and apply the cheapest postage.  I came up with the same postage fees as the clerk at the PO.  So far no problems.

http://postcalc.usps.gov/

I'm looking at a USPS chart and first class flat and package rates do not ever correspond at any point with the same weight.  Flat starts at 80 cents with 17 cents per additional ounce.  Package starts at $1.13 and same 17 cents an additional ounce.  There are quite a few times where the difference is only 1 cent apart.  But because the rate is based on weight, it can never be at the same rate in 2 different categories.  Hope that makes sense.

Any item less than 3/4 of an inch thick can qualify for "flat (large envelope)" status and cost less than a parcel.

My postal worker, who is very much on the ball, was confused by this one, because due to the low weight of my package her computer did not allow her to use anything other than the "flat" rate.  I just hope that even if the postage is off it will still go through since it is clear from the meter strip that I paid what my post office told me to pay.  Thanks for your responses.  Fingers crossed!