Hi Kathy, I did a Google search on the same topic and found your forum entry. Not being able to send invoices via Squarespace is a bummer when you are based in the EU. I'm from Switzerland and it's common to get paid by bank transfer, especially for B2B.
The automated invoice function you are looking for will only work with an e-commerce system like WooCommerce or Shopify.
If you like to go around the credit card payment fees from Squarespace you would need to set up a direct link to Stripe/PayPal for the invoice. If you like more details, please let me know.
I also found a link - this is a work around solution with Shopify: https://sf.digital/squarespace-solutions/how-to-add-afterpay-ideal-klarna-or-zip-pay-to-squarespace
I also thought about this solution because I also build Shopify shops. Maybe this helps, maybe you already found a solution.