Hi Erin
Could you let me know what version you are using ? In the latest version, version 2.6.0, the payment tab in configuration had been removed. To choose the payment method you want to use, please go to Documents Seller -> Payment Plugins and enable the payment method you want .
If you enable more than one payment methods, your customers can choose the payment methods they want for processing payments .
Hope the explanation is clear to you. Maybe you should send me your site information so that I can check it ?
Thanks,
Tuan