You pay per household, not per head
A printed invitation is addressed to a family or a couple, so 150 guests is around 75 cards, not 150. Almost every comparison published on this subject multiplies the guest count by the card price and ends up with a print column twice the real size. Here the ratio is a field you edit: if a lot of your guests come alone, set it to 1.5 and the result corrects itself.
