I am not sure if you said it right- that they only gave you 30 days to provide the KSA Police Certificate, because when I received an email from ADVO requesting for my Saudi Police Certificate, it was stated there that I have 90 days to submit the said requirement.
It is true that ADVO (not Canadian Embassy Riyadh) will send the original request letter via regular mail that is why they also mentioned in the letter to wait for maximum of 4 weeks to receive that letter.. it is via regular mail and not via courier like ARAMEX that is why it will take time for you to receive it. And giving you only 30 days to comply and submit your clearance is relatively not possible.
Once your receive the letter, you have to take it to the MOFA (ministry of foreign affairs) for approval, you have to pay the fee required as they will stamp the letter. Thereafter, take the letter with approval stamp to police headquarters depending on what region you are residing in Saudi Arabia. From there, police officer will take your finger prints and all necessary information they will be needing in order to process your clearance. It will take 7 days for them process and release your certificate. A claim stub will be provided to you.
I hope this will somehow help you and give you a clear picture on how to obtain Saudi Police Clearance.