How to sell downloadable media with custom payment option

I’m not a professional developer. I want to sell ebooks on my website http://www.bond.uz and I’m having difficulty with payment options.
I tried plugins but payment options such as PayPal or VISA don’t suite my customers, because they don’t use those services. We have a PayPal alternative here, http://www.websum.uz , and I got 3 merchant php files form them: inbox.php, fail.php, success.php but I don’t know how to set them up so that I can use their service.
I want some sort of a “pay-per-download” plugin with the option to include WebSum as the primary payment option.
Can anyone assist me through this issue, please.