There are some things to consider regarding the idea of paying upfront or annual plans.
First, it's hard to predict how many models are going to get sold annually on a new unfamiliar platform, therefore a percentage of royalty is a safer bet, especially when it is as low as here on CGtrader.
Also a system that enables better royalty's when users do well and stay loyal to the platform is more interesting + if the selling platform gets a percentage on a sale it will also guarantee full incentive to strive in maximizing sales from both sides.
On other hand when one would pay upfront via annual plan then that would create some sort of disconnect in this situation.
For example, suppose a user would select a +150 models plan because he usually sells +/-300 models a year on an established platform, the +150 models plan would cost him 250 dollars, but then in that following year the unfamiliar platform only manages to sell 50 models for some reason?
The user would have paid allot of royalty and the selling platform losses noting, that place would have collected the money regardless of the poor sales performance.
In short, it's probably better to opt for giving a percentage on a per model sale and 10 to 15% is a very good deal.