Thanks for the replies.
My wife did call the post office. They advised us to come in so that the letter could be weighed and an accurate amount calculated.
RD, the invitation is to our niece and her husband who are living in Toronto. Our youngest son (a young soon to be 38!) is getting married Labor Day week end.
As well, my wife is inviting her to a shower for the bride, so we will be sending her an invitation to that.
...and already the clothes shopping has begun....we will be spending the majority of the week end visiting various stores so my wife can get what I am told is required for A.) The bridal shower; B.) the rehearal dinner; and C.) the wedding it self. I can hardly wait. Thank goodness for the portability of Kindles....