Hello. I am also having this problem. I contacted SS customer service over chat and they told me:
it looks like an issue with the connection between Stripe and Squarespace is causing unexpected behavior with the checkout page. From what we can see, Stripe is showing as connected on the site so it may have been disconnected on Stripe's end somehow.
To reestablish the connection, could you try disconnecting and reconnecting Stripe on the Squarespace site as outlined in our guide here?
https://support.squarespace.com/hc/en-us/articles/206540917-Accepting-credit-cards-with-Stripe
As long as the Stripe account is connected properly, the checkout process should work as expected on the Squarespace site.
Hopefull this helps you guys with your sites!