What is the word for when you don't notice something?

"Oblivious" used to refer to forgetting, but is now often used for this sort of failure to notice.

Is polite inform?

"I would like to inform you" is quite formal-sounding but might be appropriate, depending on the client. You could also say something like "We wish to notify you..." or "We wish to let you know...". The formality really depends on your relationship.