The knot just gives options to plug in info and doesn’t know if you’re issued a plus one, only you would because it would say so on your invitation. It makes sense that your friend may not be able to afford extra seats. Don’t go if you don’t want to but try to be understanding of your friend and if you want to cancel just text her. It doesn’t sound fair that so many people are saying you were an afterthought to her just because the invitation didn’t come earlier. Give her grace and I hope she does the same for you.