Help in the sentence usage of Since and as
Kindly see the text that i am writing to officials to renew my Insurance policy
"As I was out of station for an unavoidable work, I have not paid my insurance due as on date. Hence I request you to kindly consider my request to update my payment that i am making now and make my continuity from renewal date without any gap".
is this right?, should i write "as" or "since" in the beginning?. I would be thankful to correct the sentence or replace the whole if you feel it is inappropriate.