Started using Zwift about a week ago and love it. After just a couple rides I wanted to upgrade - looked for a way to upgrade past the free trial period but couldn’t see an easy way to do so wherever it told me I had x kilometers left on my free trial.
I started a workout this morning and saw that I had about 25k remaining on my free trial. Got most of the workout done fortunately but during my last set got a pop-up saying I needed to upgrade. So I jumped off the bike and scrambled to try and enter in my credit card info as quickly as possible - meanwhile, my rider seemed to keep riding behind the pop-up window despite me no longer pedaling, so the stats on my ride now are a bit off. Finally, when I got the CC info in and upgraded from the free trial, I was unable to get back into my workout and finish it.
A good general rule would be to give a user the way to upgrade to premium whenever and wherever you are showing them how much time/distance they have left with the free trial. You may even get a bump in early upgrades from this, and avoid similar scenarios to the one I encountered.
Overall amazing product though - love what you all are doing, keep it up!