Ask HN: Recommend a payment platform for my saas project?

I'm ready to add subscriptions to my consumer saas project. It is a Gmail addon accessed through a web app and chrome extension.

I have a fremium model, with multiple levels of upgrades.

Can anyone recommend a solid payment platform that a one person dev can implement with minimal fuss?

I need subscription support (monthly, annual), security, and straightforward apis. Also, I don't want to store any card numbers in my app.

Is anyone using chargify to handle the account management part? I like the feature set but it gets expensive as you scale. I could spend time upfront to code those features myself.

My app is written in Python running on Google app engine.

Thank you.