Syndicate RPG credit shop is the ideal way to monetize your Synserver. It allows players to donate to your server for a reward without making the gamemode pay-to-win. You can also sell items directly if you really want to.
The script adds an button to the players HUD. Once they press the button, the credit shop will show and they'll be able to donate to your server!
This system is a layer between your donation system and the player (meaning you still need one). All are compatible (console command support).