While it’s more formal and traditional to use an inner and outer envelope for your wedding invitations, it’s more common now to use a single envelope. Following are some pros and cons.
The outer envelope will protect your invitation in the mail.
You can use a liner on the inner envelope.
It is a nicer presentation.
It increases the overall price of your invitations because:
You have to address the outer and the inner.
It increases the weight (possibly the postage) of your invitation.