Late payments

I have been paying my Comcast bill on time for years, always mailing it the day after I receive it. Yet, some months my payment is not credited until 30 days past the due date. This is unacceptable. I pay nearly all of my bills by check, and Comcast is the only company where this issue occurs. Why is that? I should not be forced to pay online simply because of repeated problems with your accounting process. How can this be corrected ? I do not want to pay my bill online and I should have to do so because of the issue on your end.