Calligraphy is a kind of music not for the ears, but for the eyes.
V. Lazursky

My philosophy is for calligraphy to reflect your genuine style.  Your invitation provides inspiration to create a font that brings the invitation and calligraphy together.  By capturing elements that you love about the invitation, maybe it is the scrolls or the polka dots, and incorporating that into addressing the envelope makes for truly one a kind calligraphy.