The Xiamen Waidai and Fuzhou Waidai you mentioned are local branches or offices of China Waidai. The full name of Waidai is Ocean Shipping Agency, which is a shipping agency. Foreigner, Sinotrans and United Shipping Agency are the three largest shipping agencies in China, which basically represent most shipping companies.
Shipping agents help shipping companies to handle some port business and documents, and connect the handover between port areas and shipping companies. For example, issuing export documents to the dock to receive goods, and acting as an agent to import foreign exchange documents.
A shipping agent can represent several shipping companies, and the shipping company is Party A of the shipping agent. Moreover, a shipping company may divide it into different shipping agents according to different routes. There are also some shipping companies that do everything by themselves without shipping agents, such as China Shipping.