The payment is currently not dependent on NewGRFs. Instead it is just a constant equal for all cargo types. You can change this in the GS settings window.
- Pay companies monthly based on cargo delivery past month
- Reverses OpenTTD default income by the end of quarter (can be a bit later as it happens in the next monthly update)
- All money transfers by this script are shown on the other row in the finances window
- AIs can identify this GS and get the payment factor using SCP.
Expect that AIs in general will have problem with this GS, as they need to specifically write code to be aware of if this run in your game and then act differently based on that.
Edit: It requires OpenTTD 1.4+ to work.