How are commissions set between Storefronts and Suppliers?
Suppliers set a commission when they sign up to Canal (usually ~30%). This is the percentage Suppliers are willing to pay Storefronts for sales of their products. Storefronts who elect to work with a given Supplier must accept that commission.
For each proposal Storefronts send to Suppliers, Suppliers have the ability to accept, decline, or offer a counter proposal.
To understand Canal's commission rates: See FAQ on Canal's Commission Rates
Can a Supplier set commissions on a per-product basis, or do they receive the same commission for every product?
Yes, Suppliers have the ability to vary their commission per product.