How can I manually send order details email to customer in Ubercart? I have a cron job to read orders from the Ubercart database table, and manually send the order email to the customer.

Usually, when customers checkout in Ubercart, they will get their order email straight away. But in my situation we don't want to automatically send the order to customer because we have some process that needs to be completed before we programmatically send the order email to customers.

How can I manually send the customer's order email in Ubercart?
You can try hook_cron in a custom module and get all orders using uc_order_status_list the return value may contain the email address of the buyer. If not get the order id, use uc_order_load to load the order. Get the email of buyer. 

And you're done.