It is not "illegal" in the sense of being against a law, since there are no laws saying how many days a service like this should be provided, or that a truck has to be dedicated. It is a breach of the contract, however, between the two companies, if your former company is not giving the client/customer what they agreed to and what the client/customer paid for. The client/customer could sue your former employer for "breach of contract": for not honoring their agreement.