you can think of it like this.
You are standing on the street and need to get to the other part of town. You can go to a bus stop, sit on a bus and get there, but this probably can take a lot of time
You are standing on a railway station in Paris and you have a piece of paper that you need to be delivered in London. The difference is that all of the train passengers are couriers who deliver documents.
In a centralized world, you would just go to DHL, pay a fixed fee and go away. This is how it goes usually, you don't mind how much was the actual cost of sending this document.
But in the blockchain, you actually can just stand in the middle of a train station with a poster:"Please deliver a piece of paper to London for $xx".
Every courier (miner) has a choice to grab your document with them. If you set a very high price, probably the first passing courier will pick up your document.
If you set a very low one, eventually you will get it delivered, but more probably on a weekend or at night, when there is not much business going.
Wallets with fixed commissions are like courier services. And for paper wallets, for example, you state the amount yourself.
But keep in mind, that in general transaction fee does not depend on the amount you want to send. Like the delivery for a piece of paper does not depend on the text printed on it. On small transactions, this can result in paying 10-25% commission.