What should I watch out for? – Selling designs on IG

Hey just looking for some advice here.

Started posting some design work a few months ago to build up an IG following.
Recently a clothing brand startup contacted me saying they will pay me to use my designs on shirts and stuff. Basically, a percentage of every sale with my designs will go to me. (I was not really expecting this so early)

They want me to make like 10 designs they can look through and I agreed because It seems like a decent enough deal and I enjoy making designs anyways….

What should I watch out for? – Selling designs on IG