I found some PHP in the Formidable Forms Knowledge Base that shows how to remove 'line2' from the Address Field using frm_address_sub_fields. I changed this to remove the other fields, leaving only the zip/postcode field.
I've tested it, and it seems to have transacted properly in Stripes test mode, I will find out later today from the client if everything looks good on their Stripe Test Dashboard, but in the backend of the Formidable Forms plugin, the payment entry looks good to me.Below is the adapted code I used.

add_filter( 'frm_address_sub_fields', 'customize_address_field', 10, 2 );
function customize_address_field( $sub_fields, $field ) {
	$target_field_id = 262; // Change this to the ID of your address field.
	
	if ( $target_field_id !== (int) $field['id'] ) {
		return $sub_fields;
	}

	// Remove the fields we don't want.
	unset( $sub_fields['line1'] );
	unset( $sub_fields['line2'] );
	unset( $sub_fields['city'] );
	unset( $sub_fields['state'] );

	return $sub_fields;
}