Can a doctor's office charge you a fee if you don't pay the full amount within 25 days?

Question Details: I recently started going to a endocrinologist here in Oregon for ongoing care for thyroid cancer. Before they would agree to see me, I had to sign a paper saying that if I no-showed or cancelled my appointment with less than 24 hours notice for any reason, I would be charged the full cost of the appointment it was something like $300 for regular 15 minute appointment, $500 for 30 minutes, and $2,500 for the procedure appointment. I signed with hesitation because I needed care quickly and I never no-show appointments. It just struck me as odd because we have inclement weather sometimes here that is out of my control but I agreed nonetheless. I've had a couple appointments and I just received a bill that says if I don't pay the full amount owed within 25 days, they will add a $25 late fee to my account. Luckily, I have the money at the moment, but there are time when my medical bills are so high that I can't pay everything right away. Is charging a late fee after only 25 days for a medical bill legal?

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