Give your merchants a list queue that they have to send to. TradeEx send resources to villages every specified interval time. TradeEx doesn't work in order, so they aren't be affected by position of items in queue.
Interval is an integer representing the minutes, not representing the seconds.
Queue Items can be added by clicking on "+TradeEx" button of village's detail page. Click "Edit" to begin editing amount of resources, then click "Check" to end editing.
You have to click Save button after adding items. Checking before save is necessary if you want to avoid mistakes.