Customs duties charged by FedEx

Both FedEx and UPS have this phenomenon. There is no problem collecting my tax on the watch accessories I bought from abroad, but they can't give the tax bill issued by the customs, saying that it is declared in batches, and there is no tax bill, and the invoicing is also a service invoice issued by them. The tax rate of general service invoices is 6%, while the tariff rate we charge is 25%, which is 19% tax. If our parcels are brought in, the two express delivery companies in the United States are worse than China Post, with high fees and vague terms, but they have not been investigated by the customs department. What's the problem?