Alternative idea:

Bring a piece of paper along with you on your visit to the medical provider’s office bearing the following statement (or words to its effect): “_________________ [clinic, hospital, doctor, whatever] avows that the amount shown on invoice #________ represents the sum total of all charges related to the procedure(s) listed, and _______________ [clinic, hospital, doctor, whatever] holds patient harmless from any future charges that might be levied from other entities (such as testing laboratories) related to or arising from the services listed on invoice #_______.”

Hand the piece of paper to the admissions clerk at the time you agree to be seen, explaining that you will refuse to pay any invoice unless you get a legally binding signature on your piece of paper prior to or contemporary with the presentation of their invoice. Explain that their acceptance of you as a patient constitutes their agreement to your terms, including your non-payment if they fail to provide you with the signature required.