They charge 0.5% per invoice for the higher tier, so for 20k that would be 100 bucks. Which is still excessive imo, but not 500 bucks.
The rationale is that Stripe believe that a small percentage will align better with customer acceptance of pricing than a flat fee per invoice, which allows them to capture more dollars from the relationship.
The cost of transaction is the same. Customers are being robbed in day light with % based transaction fees since the dawn of time.
This entire industry is begging for disruption.
Those costs go up with the value of the transaction.
See Atlantic Monthly vs Wise for different pricing types in fx transactions as an example.
There is an additional payments fee that applies.