Add online payment module

Write an online payment module (sber) in addition to the existing one (you can copy, with a change in the merchant) and place it on a separate button.Site on Drupal 7.
For the correct operation of this module, it is necessary to change (refine) the database of the site (mysql/ MariaDB) in order to: obtain data to use this particular method of payment, fill in some data (name and several digits) in the database of the program (not the site), check the presence of these data in the database of the program and when finding a duplicate - do not allow this payment method.
The budget is very approximate, agreed with the performer.