Same problem here...
I try to create a Click & Collect module for my restaurant in France and I want my customers to have the choice between paying online or direct at my restaurant's doors.
But, I may found a simple solution to avoid oneline payments for Stripe users (only). It cans help until Squarespace offer a better solution.
=> In the "payments " page, enable the "test mode". Once enable, no payment will be ask when you finalise the checkout.
Then, just edit a comment to explain to your customers that payement will be made on site via cash, check or credit card.
Sorry for my English...Not perfect :-)