All the delivery rules are set by the producer in the admin. Producers who have opted to provide delivery, set the fees and schedule along with processing time. This information can be found on every product profile, in the 'Fulfillment' tab.
Our system calculates the delivery fee based on the rates set by the producer (if self-delivering). Any delivery fees are paid by the buyer (unless sellers have specified over-wise) and all fees are processed as one transaction at the checkout.