Default Paid From when creating a purchase

Something is probably doable with some custom JavaScript, if you have a Power User subscription:

jQuery(function($) {
  if(document.body.classList.contains("page-purchases-create")) {
    const bankSelect = $("#dropPurBank");
    // when I tick the "paid" checkbox...
    $("#chkpaid").on("change", function(e) {
      // and the currently selected bank account is the default current account...
      if(e.target.checked && bankSelect.val() === "1200-GBP") {
        const supplierId = new URL($("#hrefSelectedSupplierName").prop("href")).searchParams.get("sID");
        // and the currently-selected supplier is the one I'm interested in...
        if(supplierId === "123456") {
          // then select a different bank account in the drop-down
          bankSelect.val("1206-GBP");
        }
      }
    });
  }
});

You’d need to replace 123456 with the supplier ID number of your Stripe fees supplier (from the URL of the /suppliers/detail?sID=<number-here> page) and 1206-GBP with the appropriate nominal code of the account you want to select.