In the "text" step of your card customisation, you will see "First Name Last Name" by default on the front of your envelope. Please note that at this stage, it is not possible to preview each recipient's name on the envelope. The names you add to your contact list (Sending) will automatically display on the front of the envelope.
To modify the text on the back of the envelope, scroll down to the "back text" option in the same menu.
In both cases, you have the option to customise the font of the text on the envelope. From the customisation tool, click on the menu on the left to choose the typography, color, and size of your font. The changes will be applied immediately.