You can select the font used for the return address by clicking the Fonts button. When selecting the font for the address, the font selection applies only to the lines of the address selected. Thus, to set the same font for the entire address, select text in all lines of the address (which is the default setting) and then select the font.
To set the font for a single line either select text in only one line or place the cursor in the line (with no text selected) and then click the Fonts button.
Your return address can be in any font. However, if your mailpiece is undeliverable, the more accurate and legible that your return address is, the more likely you will get your mail back.