Instacert does NOT guarantee that you will pass -- that would be a different online group, one that I would not suggest to you. Nobody can guarantee that you will get a score of 75-80 unless they stole the questions that they know will be on the exam. Also, I hope you realize that a perfect score on the CLEP exams is 80. Their scoring is a scaled scoring, not a percent.
Instacert uses a flashcard system of study - these flashcards are not meant to suggest these are the questions you will see on your exam. Rather they allow the testing candidate to see if they really do know the material that will be covered, how prepared are you for the areas that will be on the test.
Let me suggest another website to you (if I didn't already) -- www. free-clep-prep.com -- as this site has free study guides along with suggested online resources.
You keep questioning whether AP material is good to use. Does this material cover what the CLEP website says will be on the biology and chemistry exams? If it covers some of the subjects, but not all, you will need to add in some other resource. Freeclepprep and Instacert (check out the General Education forum and do a search on biology and chemistry) have good suggestions for study materials.