NO ... do not mail anything if you pay online ... they know you paid ... they do not need proof. And it is way too late to file an extension for 2018 or 2019. If your 2019 extension payment did not go thru with the extension it is too late to pay or mail it in ... you only had until midnight on 7/20/20 to "perfect" the rejected extension and payment so now it is just late.
