If a package is not using the new package pricing model, based on the retail prices of the individual products contained in the package, it cannot be purchased through the Shopping Cart. Partners can update their packages to use the new pricing model at any time. Note that packages using the old pricing model also cannot be added to an invoice.
In the new package pricing model, the Purchase Behaviour setting allows Partners to specify the behavior and messaging of the button in the Pricing section of a package in their Store.
Select Add to Shopping Cart under this setting to allow your customers to add a specific package to their Shopping Cart.