With that horrible grammar, I can see why you're getting implants.
He may consider a check the same as cash just not accept credit cards or carry credit for customers. Cash isn't shady unless he isn't keeping his books correctly.
Many places prefer that because people who get breast implants are probably the same who will phone up their credit card claiming faulty products and demanding the charges be reversed, which of course can't easily be done. Plus, the doctor loses 3% on the credit card transaction, which is money out of his pocket. If you really want to have a paper trail, write a cheque. If he doesn't accept that, yes, that's sketchy.
that sounds incredibly shady.