How can a doctors office charge you his fee and his billing service charge another fee?

If I paid for my doctors service fee in full, with no balance, how come his billing service he hired, can send me another bill for their service to the doctor. Isn't that part of His overhead cost and not my respondsibiity to pa a separate fee?|||More info - what is the billing service billing you for specifically (does the bill say "Billing Service Fee" or something else). Do you have insurance - if so, when you go in, you may be paying the co-pay, which would not cover your portion of any tests or other services provided, in which case, the billing service is billing you for the portion that insurance did not pay (less agreed upon discount).