The payment of fees in return for goods or services is literally the dictionary definition of “customer”.
Ships come into port and load/unload cargo using infrastructure provided by stevedores for their benefit. Shipping companies pay fees to do so. Shipping companies are customers of stevedores.
Trucks come into port and load/unload cargo using infrastructure provided by stevedores for their benefit. Trucking companies pay fees to do so....
https://shippingaustralia.com.au/shipping-and-trucking-companies-are-both-customers-of-seaports/