Invitation / card is not centered, or the inner side doesn't fit the outer side:
Open printer properties window.
Make sure the defined paper size fits your actual paper size (A4 or letter) 
Invitation/ Card edges are cut off or the card printed landscaped :
Open printer properties window.
Make sure that the invitation orientation selected is “Portrait”. 
Greeting card inside page is printed upside down on duplex printers
On print settings under 'Two-Sided Printing', make sure that 'Flip on long edge' is selected.
Low quality of printing
Choose the highest quality printing that your printer is capable of (‘Best,’ ‘Fine,’ etc.)
Make sure to select color printing, not black & white.
Heavyweight paper, such as card stock, is recommended.