You can pay the market price for power or agree on a fixed-term fixed-price contract. In the latter case, the power company carries the risks. The market is cheaper on the average, especially if you can adjust your demand down when the prices are high. Back then, the prices were relatively stable, but today they fluctuate wildly due to the prevalence of wind power.

In any case, carbon-free power from the grid is mostly a legal/financial construct with limited real-world impact. When the grid is dirtiest, market prices are set by fossil fuels with high marginal costs, and clean sources get the same price without any tricks.

Perhaps one company buying 24/7 CFE won't make much of a difference in Europe, but it would make a difference in places like Singapore, where the existing grid is incapable of providing 24/7 CFE to anyone.