Many times, even though it doesn't look like it, your stationery or invitation may require extra postage. DavidGregory.biz warns you if this may be the case, but the best and safest method is to take your fully stuffed envelope to the post office and have them give you the lowdown. If you are absolutely against this, try out the Post Office's extra postage formula (as of 11/97). Any card will require extra postage if the measurements exceed any of the following: 6 1/8 inches of width, 11 1/2 inches of length, or 1/4 inch of thickness. Also, additional postage is needed for any piece whose length divided by height is less than 1.3 or more than 2.5.