The messaging API is the API to build bot application. So you can't use the Messaging API without BOT. For detail, please see this documentation. https://developers.line.biz/en/services/messaging-api/
the user is able to chat with our customer service on LINE app
If your "customer service" is BOT account (LINE@), it is possible by using this URL scheme. https://developers.line.biz/en/docs/messaging-api/using-line-url-scheme/#sending-text-messages However, if you don't mean BOT but normal LINE account, the URL scheme doesn't work due to security reason.