I think it adds to 8%. This page [1] makes it clearer. They charge 5% and their “payment processor also charges a processing fee” which in the US is 3%.
The tip seems to be something entirely separate, which is _additional_ money, on top of your donation, that only goes to IGG, based on another [2] article.
You probably have more experience here though. Your work is great and I always appreciate your posts.
I made an $11 donation, and Indigogo used a dark pattern, automatically selecting it, to try to make me "tip" Indigogo $10 on top of that! A 90.9% "tip!" NO THANKS.
A tip is for a service rendered. Not for being a mendicant middleman.
And when you donate you can set the "tip" to 0% so all your donation goes to the org.