A 10.000+ container vessel uses 100-120 tons of fuel per day (see https://people.hofstra.edu/geotrans/eng/ch8en/conc8en/fuel_c...). Bigger ones are more efficient per container, but it'll still be more than 120 tons of fuel. If you fuel up, that's a lot of bunkers to take in. That's not paid in cash. Credit is used.