You can't ship anything with DC if it is not classed as a First Class Mail Parcel

Ive been smacked with that rule twice myself. It depends on what kind of a day the clerk who runs them in to the system is having I think.

After the second time I started making sure I tossed in a packing peanut or wad of bubble wrap just to make sure it was thick enough.

Both times I did go back to the post office, point out their own rules, and ask to speak to the post master who had to agree with me, unfortunately, because they had cancled my postage, I still had to pay again *Sigh*