I don't believe there is an official AP position on "handle with care" "do not bend" or "fragile" written on a letter/parcel.  Maybe there used to be a FRAGILE sticker that meant something previously.

Some people might say that noting these on an item causes the handler to do the opposite but I doubt it.

However, there are many very good posties around who will ensure at the very least that a "do not bend" item is not folded for the letterbox.

So if a buyer has requested "do not bend", make sure it is very clear as well as ensuring it is reinforced to make sure bending it is not easy.