Email is checked case sensitively in uc_cart_complete_sale() function

$result = db_query("SELECT uid FROM {users} WHERE mail = '%s'", $order->primary_email);

This check will fail if database compares strings in case sensitive manner. Must be
WHERE LOWER(mail) = LOWER('%s')

There's one more similar check in uc_order_select_customer() function:

$result = db_query("SELECT uid, mail FROM {users} WHERE mail = '%s'", $email);