What are the best set of marketing/payment tools for use in startup with a subscription webapp?
Stripe has excellent support for subscription payments, including helping manage recurring payments and pro-rataing for upgrades to plans half way through a billing cycle etc. I believe BrainTree offers similar tools.
For email stuff, I strongly recommend looking in to Mailgun—they offer fantastic tools for advanced email sending. They don’t have the same user-level engagement tools as something like Mixpanel or Intercom though.