If you write the cards upside down, you are saying the picture on the front/the printed writing inside the card will be upside down when you write your own words on the card. Why would anyone do that? Did you mean someone else?
If you were to say "I write my Christmas cards upside down" that could mean you yourself were upside down when writing the cards! If you are into yoga or some such activity you might like to so that. However, it is very unlikely. Therefore, saying you wrote your Christmas cards upside down would be taken as how you wrote them rather than the physical contortions involved in writing them! Hope that's clearer still now.
@BluePeter ( I tried to explain what’s actually happened, but apparently it wasn’t posted, an error had happened) that stupid person was me actually 😂 I wrote loads of Christmas cards and everything went perfect and I mistakenly wrote the last card upside down and I didn’t have anymore cards left and I was going to see that person so I didn’t have time to buy another card, I thought I’ll give it to her and I explained what’s actually happened. It was really funny 😂, so in this case is “ I wrote the Xmas card upside down “ correct?