The plugin offers you to accept payments via Epay.bg and EasyPay, which are the most popular payment providers in Bulgaria. Please refer to the Readme.txt file for a link to the Epay.bg documentation.
The payment plugin integrates on the Payment Method step of the checkout page. The plugin displays the Epay or EasyPay payment methods based on whether the store owner has enabled them in the plugin settings from the configuration page.
Once the checkout is complete the client will be redirected to either the EasyPay or Epay page in order for them to enter their payment details. After that, the client will be redirected back to the store and either the Order Completed page or the payment method error page will be shown to them informing them whether the payment was successful.
Find the plugin's Github repository here