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.