Hi @Hai Ta -- Cam from the Intercom Support team here 

Have you had a look at using Series to achieve this? It sounds like it might be a great solution given it can allow the end-user to enter the Series on the day they meet a specific criteria (subscribe/upgrade to ‘Premium’) and then uses a Wait Block to hold the end-user within the series for the desired timeframe before sending them an Outbound - Email. I threw together a quick example Series to show the rough idea of how you could do this :

Hi Cam, thanks for the answer. Will keep the Series message in mind for future reference. I found that the solution is also in the Email message itself. I was just playing with the wrong component. I should have been editing the Audience criteria, instead of Trigger. Because in Audience criteria, there can be a time delay based on the Stripe attributes.