This forum is not actively monitored for product feature requests so to provide feedback about this to Squarespace, raise a ticket through Customer Care. This will add your voice to the requests for this feature and it will be shared with the appropriate team.
In the meantime, there's only one workaround that I know to be successful and that's to buy a $9 per month solution called 'Shopify Buy Buttons'. I wrote a quick guide that explains how to add these to Squarespace so that you can use Shopify's commerce features on a Squarespace website: How to add Afterpay, iDEAL, Klarna or Zip Pay to Squarespace
Update February 2021: Squarespace added Afterpay as an option in the US, Australia and New Zealand. Canada and the United Kingdom will follow later.