Envelope design

Now that I am happy that my cards are coming together, I have decided to focus on my envelope design. Because my cards are quite thick I have decided that they would suit being boxed rather than put in a paper envelope, however nothing beats the excitement of getting a letter through the post around your birthday, so I have decided to keep the appearance of the letter and put this on my box design.
The box will need to be thin light weight cardboard that will keep its shape and protect the card but will not add to the postage price. 
The box will need to be fitted snug to the card so that it gives maximum protection and does not let the card bash the sides, however it can not be tight as the card will need to be inserted and removed with ease. To make this process easier I have decided to make sure the box opens on both the left and right hand side so that there will be no issues of the card getting stuck.
Here are the front and back designs for what I would like the box to look like.