Squarespace Single-Checkout Subscription Integration

I run a Squarespace store that sells software licence plans. Each plan needs two charges:

1. A one-time upfront fee that covers setup, the first month, and a reserve
2. An ongoing monthly subscription billed through Stripe

Squarespace’s native flow forces customers to press two different buttons and complete two separate checkouts, one for the upfront payment, another to start the subscription. I want a single, seamless checkout where the shopper pays the initial fee and authorises the recurring charge in the very same flow.

The site is already connected to Stripe, and I’m happy to keep the standard Squarespace design; this is purely a functional/code change. Your task is to inject custom code (custom checkout, Squarespace Commerce APIs, Stripe APIs or the best proven method) so a customer:
• selects a plan
• reviews one combined order summary
• completes one payment step that captures the upfront amount and activates the monthly subscription automatically

Acceptance will be a live test transaction that shows both the correct upfront charge and the correctly scheduled subscription in Stripe, plus brief hand-off notes so I can maintain it in the future.

If you’ve built similar single-checkout subscription solutions on Squarespace (or have strong Stripe API experience) I’d love to see an example.