Here is your holy grail for info http://www.portofsandiego.org/environment/copper-reduction-program/policy-development-and-legislation/in-water-hull-cleaning-policy.html

The frequency of cleaning on a schedule is "optional" and is at discretion of the hull cleaning company. Yours is clearly trying to scam you. I'd pay only for the cleanings that were actually performed and find another company.

You can also do your own cleaning if it doesn't result "in a visible paint plume or cloud".