In this wonderful modern age you can simply write her name on the envelope. It doesn't need to label her as married, unmarried, engaged, solo, etc.
For example: If the person's name is Giselle Renoir and you want to invite her to a party then simply address the envelope to Giselle Renoir at her address. If you want to invite her husband too then simply address it to Giselle and Pierre Renoir.
The way to address a shower invitation to a married women would be you address them by their full married name. Example: Mrs. Kate Bolton.Another PerspectiveThe best way to address a shower invitation to any woman is to simply use her name. This is the twenty first century. A shower invitation does not need to reflect a woman's marital status.To: Kate Bolton.
If you are unaware whether a woman is married or not, use Ms. If it is a man, obviously, Mr. is used in all cases. so Ms. for a woman, and Mr. for a man.
you can do it either way but when i got married i did it the Mrs. Michael Smith way.
Frau
"madame"
Madame.
Mrs. is a title of respect that may be used to address a married woman. Ms. is preferable, especially if you do not know what the woman's preferred title is, or if you do not know the marital status of the woman.
Frau
Unless the woman specifies otherwise, always address correspondence to Ms.
You can address an unmarried woman as "Ms." just as well as a married woman. In a situation where you are unsure of the marital status, "Ms." is a safe form of address, and avoid "Miss" and "Mrs."
You address the envelope to the person and only put "X, Y and guest" on the invitation.
* Yes, the name and address on a wedding invitation should be centered on the envelope and your return address on the upper left.