No, you can't do a purchase order instead of paying by credit cards because this payment hasn't been enabled on the developer's website. I have simulated a purchase operation, and the only way to pay for the application is through a credit card (Visa, MasterCard, Amex). There are no other payments available for the software.
Use the info on the following page to get in touch with the developer. Maybe he can provide another payment solution if you don't want to use the credit card: http://www.sigmaplot.com/contact/
