Create up to 50 sales orders in one call. The whole call costs a single request against
your rate limit no matter how many orders it carries, so a backfill that would otherwise
need one request per order fits in a fraction of the quota.

Every order is validated and stored on its own: one rejected order does not hold back the
rest of the batch, and nothing another order does can roll it back. The response carries
one result per submitted order, in the order they were sent, each tagged with the index
it had in orders so it can be matched back to the payload.

As long as the batch itself is well formed the response is a 200 - even when every order
inside it was rejected. Read meta and the per-order status rather than the HTTP status
code alone. A 400 means the batch was never processed at all: orders was missing, empty
or held more than 50 entries.

seller_order_id still has to be unique for its channel, and that includes within a single
batch: send the same pair twice and the second one is rejected with error code 4003.