Change the rate over a customer's lifetime, for example 30% for the first 3 months, 25% until month 5, and 20% after that. Create one flow per period, each with a Created date condition on the customer.
Set up the flows
Create a flow with the New payment trigger and a 30% commission. In step 4, add the customer condition Created date is within 3 months.
Create a second flow with a 25% commission. Add Created date is within 5 months, click AND, and add Created date is after 3 months.
Create a third flow with a 20% commission and the condition Created date is after 5 months.
Each payment now matches exactly one flow, based on how long ago the customer signed up, so the rate steps down on its own.
Good to know
Keep the periods from overlapping. If two flows match the same payment, the partner earns both.
To count from when the customer first paid instead of when they signed up, use Conversion date.
The Time-based commissions template sets up the first flow for you. See Create flows from templates.