Commissions can be applied in 2 different places in the Business Rules.
1) Pricing Rule Adjustment
In the Business Rules add a Pricing Adjustment Rule to cover the Commission percentage that will be applied to all items in the price book that are using Calculated pricing.
**IMPORTANT - pricing adjustments will only apply to items that are using calculated pricing. Contractors will commonly carry services and accessories in their price book at Cash Price. If this is the case, the cash price should include a % increase to cover the cost of commissions.**
The Commission will be applied after the margin or markup has been applied.
2) Include commissions in the margin/markup percent applied.
Increase your desired margin by the commission rate.