Hi Tara,
Which version of the Cart and DNN are you currently running?
In order to help troubleshoot this issue please provide the following:
- A screen shot of your BuyNow settings; General, Shipping and Tax sections
- Screenshots of the setup screen of the Products in question you are calculating tax on
- The "From" and "To" shipping addresses of the order
- A screen shot of the Cart page after you have calculated tax showing the tax applied
Thanks