Add a warning email 24 hours before the transaction. In the subject put "You'll be charged $10 for renewal in 24 hours".

> we also provide a simple, self-service way to cancel the subscription

It's hard to comment without seeing screenshots and description of the process. When people are cancelling under a perceived threat (will be charged otherwise), any inconvenience will become an extreme one.

Email about an upcoming charge is automatically sent a couple of days in advance, that’s Stripe standard feature. The other question is whether the user actually check their email (or which email they registered with).

As for cancellation, the problem is that even though it’s easy to cancel a subscription, chargebacks usually happen afterwards. It doesn’t matter how simple cancellation is, what matters is that the user first cancel and then still goes and files a chargeback.