Split flow + commission

How a split is meant to be worked out for each seller in an order:

1. Take the seller's items total.
2. Your commission = that total × the seller's own commission rate (on the seller's profile).
3. Seller's share = total − commission.
4. With Marketplace Collects, the whole amount is recorded as collected by you; with Direct to Vendor, only your commission is.

Note that this uses the commission rate on the seller's profile, not your Commission Rules — so the two could disagree. Today the split isn't recorded at all; the Commission Rules in the Commissions module are what count.