For payment that complete during the checkout (Paypal, Stripe) the
amount was recorded twice against `order.payment_total`. This is because
the `payment_total` gets updated in an afer_save Payment callback when a
payment is completed, and then once more when we process payment in
`Spree::Order#process_each_payment`.
This is an existing issue on master, but it was hidden by the
`update_shipping_fees!` callback, it trigerred an update of the order's
total, which then updated `order.payment_total` with the correct value.
Now that we removed the callback, the bug showed up.
Note, I updated the stripe specs for consistency even though they are
currently disabled.
c71ae35685