My customer made two separate orders and wants to combine them.

Unfortunately, combining orders isn't possible. Once an order is sent to the lab, it gets a unique ID number that the lab then uses to process it. So if your customer has placed another order, each will have its own unique lab ID, and they will be handled separately by the lab. This is a standard process for all photo labs, and GotPhoto is unable to modify or combine orders once they're in the lab's system.