Does iOS UnitySendMessage queue up whilst loading scenes?

I have written an Objective-C plugin that receives app store transactions. After a transaction is successfully received UnitySendMessage is called in order to send the transaction information to Unity.

I had assumed that this would run in the main thread as the class is instantiated from a unity class. This means that scene loading would hang the application completely. This would make UnitySendMessage safe if a transaction is received whilst a scene was loading.

However, if this is not the case and the transaction handling class is running in a different thread somehow then will UnitySendMessage queue up messages until the next Update cycle?

I need to know if I need to implement my own queueing of transactions in this case.