Proper in One Culture but not the Other...

What do you do in this case? As you readers probaly know, I was raised/exposed primarily to western culture, especially when it comes to manners. I only recently found out that some things I was taught (in an Emily Post sort of way) was actually considered rude in traditional Chinese culture - especially with the elderly. For example, I was told, literally just two days ago, that it was improper to address invitations sent to my grandparents as Mr. and Mrs. It HAS to go to "Grandfather" or "Grandmother." I was taught that something like this goes into an inner envelope, if I have one (and I don't). The outer envelope, the one that the post office sees, should be Mr. and/or Mrs. or Ms. What should I do? Any advice?

It's interesting that many non-Chinese multiculturalists expect people like me to know all about Chinese etiquette. I do not. In fact, most Jook Sings do not.