My last job was in an agency that used to build websites for a fixed cost. After a few years of doing it we changed the policy so that integrations were charged

My last job was in an agency that used to build websites for a fixed cost. After a few years of doing it we changed the policy so that integrations were charged as time and materials. You just can't judge how long it's going to take when you have to deal with an external organisation. Hell, one of the projects I was working on before I left (a year ago) was started 2 years ago - and it's still going on. It's just a simple authentication integration but they're at the mercy of the other party. No actual code has been written but plenty of developer hours have been wasted - including mine.