As the depletion of IPv4 space draws nearer we have added a new requirement when placing an order which contains addtional IP addresses. In addition to the usual suspects you will need to add an additional array to your order container object: itemCategoryQuestionAnswers.
One of the SL developers, Kien Phan, was kind enough to provide the example which can be found at the bottom of this post, but for the johnny-on-the-spots out there you can get a list of the possible questions with an Object Mask of questions for SoftLayer_Product_Item_Category with a categoryId of 14(secondary IP addresses).
And if you happen to have your API username and key handy you can follow the link:
https://api.softlayer.com/rest/v3/SoftLayer_Product_Item_Category/14/getObject.xml?objectMask=questions.description
-Phil