In Settings → Payments → Credit Card Processor, select Stripe and paste your Stripe secret key. You'll find it in the Stripe Dashboard under Developers → API keys. Save your settings. You can use a Stripe restricted key as well; the default permission selections should work.
Then click Set up a card reader and follow the on-screen steps to pair your reader. After that, choose card payment at checkout and PostalPoint sends the amount to the reader.
Stripe also lets you save a customer's card for later, which enables mailbox autorenewal.
Supported Stripe Card Readers
PostalPoint supports Stripe's smart readers, but not the small phone-only ones.
Specifically, PostalPoint supports any reader listed as supporting the JavaScript or Server-driver integration option under the Integration section of its information found by clicking on it from this web page.