Texas attorney–credit issues

I write a Q&A column on small business and credit. I received this question from a reader and am looking for an attorney knowledgeable on Texas law to help me answer it: “I have a small business in Texas. I have a vendor who gives a discount of a set percentage amount if the bill is paid by the 15th of each month; however, it states on the statement, the
discount would be reduced by 2% if it is paid by credit card. Is this legal under the Texas Statute 339.001?”