In this case, after "would".

where u have it is fine

All of the following would be correct: (Certainly) If you had notified me in advance, (certainly) I (certainly) would (certainly) have (certainly) availed myself (certainly) of your invitation (certainly). However, I would say "I certainly would have" and "I would have certainly" sound the best.

I've seen the word in all of those three positions!