And the logic becomes a bit simpler.

The method `CheckoutCallbacks#valid_order_line_items?` was a duplicate
of `OrderStockCheck#valid_order_line_items?`.

Apparently, it had been extracted twice:

 * 1d074c2
 * 06eb98b

But the first commit duplicated the method while the second moved the
original declaration.

This will delay the checkout request by a few seconds if there's stock
to sync. But we minimise the chance of missing reduced stock from orders
on another platform.

We still have a gap between the checkout and placing a backorder. In
that time we can't guarantee enough stock. But let's tackle that after
the pilot.

This spec would fail on Australian systems early in the morning or in
other timezones accordingly.

We don't allow variants to have zero weight or volume. But a DFC import
in production showed that some catalogs list products with zero weight.
Despite the products having a weight, it's simpler to treat these as
items.

Instead of creating a new variant every time.