Is it illegal to charge a processing fee in Texas?

Hello, I'm curious about the legality of charging a processing fee in the state of Texas. Could you please clarify whether or not it's permissible for businesses to impose a fee on customers for processing transactions, such as credit card payments or cryptocurrency transactions? Additionally, are there any specific regulations or guidelines that businesses must adhere to when charging such fees? Thank you for your time and assistance in clarifying this matter.