Correct. You cannot add another payment method to the Squarespace checkout. If you want to use another payment method (other than PayPal, Apple Pay and payment cards) you'll need to use an alternative commerce solution, for example Shopify supports Giropay and so does Ecwid. Both services have 'buy buttons' that you can embed in a Squarespace website to allow customers to view items on your site but then buy them using the other service's commerce service. For example, How to use Shopify Buy Buttons with Squarespace.
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