I was undercharged, can they still charge my credit card for the remaining balance?

I went to a local clinic in Dallas Texas, the total was $180 but the front desk lady charged me just for $18.00. She just swiped my card on the machine and i did not use my pin number, I just got my receipt copy and and I signed it. Can they still charge me for the remaining balance without my authority or signature? I did not pay with insurance it was all on my card. I was not pleased with their service and I am glad they screwed up.