Customs forms are required for international shipments destinations and including US destinations outside of the Continental United States.
To complete customs form, simply click on the Customs button found atop the Add shipment prompt.
For cases when the Customs button isn't displayed by default, click the Toggle Customs button at the bottom of the Add shipment prompt.
Also if you working with multiple shipments (packages), in the same order, remember to split the items across your packages, respecting the weighs limits allowed by your carrier and make sure to not over-allocate items and quantities to avoid over-deduction during shipping.
International shipments must include at least 1 customs item associated to each package number.
For more information, view How to Add Customs Information to Items or contact SKULabs support.