In this update I recoded a little the code and I added new features.
Features:

• Added bar %buycraftapi_bar_monthly% placeholder. You can customize it in the config.
• Added %buycraftapi_top_donor_monthly_value% placeholder. It shows the total value of all payments of the player in the last 30 days.
• Added %buycraftapi_top_donor_current_month_value% placeholder. It shows the total value of all payments of the player in the last 30 days.
  
• Added %buycraftapi_top_donor_global_value% placeholder. It shows the total value of all payments of the player.

The last 3 placeholders can be used with plugins such LeaderHeads.
Thanks to @SirApathy by helping me with tests.