Well they actually do both, they charge by volume and percentage.
There is absolutely logic in pricing in terms of setting a pricing floor, which often comes from a combination of fixed and variable costs.
Of course you are right that there is flexibility above that, but again there's absolutely logic there as well in terms of price discrimination mechanisms.
None of this is just arbitrary. These companies put a lot of thought, and logic, into their pricing mechanisms. Get it exactly right and you make a lot of money, get it a little bit wrong and you go out of business.
There is absolutely logic in pricing in terms of setting a pricing floor, which often comes from a combination of fixed and variable costs.
Of course you are right that there is flexibility above that, but again there's absolutely logic there as well in terms of price discrimination mechanisms.
None of this is just arbitrary. These companies put a lot of thought, and logic, into their pricing mechanisms. Get it exactly right and you make a lot of money, get it a little bit wrong and you go out of business.