If you are trying to create a Keap order via the API and wondering how to add taxes to the order because taxes aren’t being added automatically at the Infusionsoft/Keap/Max site, then look no further.
This can be done by adding a tax line item (ItemType: 2 on the OrderItem table) and setting the value upon creation. You should be using the tax amount presented to your customer through your storefront or point of sale, as to make sure the correct/expected amount is being charged. If you are not collecting these orders through a storefront, you will want to have something built into your integration to calculate the tax amounts during the integration step before the data is passed to Infusionsoft/Keap via the API.
Hope that helps someone trying to figure out why there is no documentation on creating orders with tax (not subscriptions).