Hmm, that is strange! Are there any differences between the locations of your customer and the test order you set up? Even though you have the option to charge tax on the product, if you do not have taxes set up to charge for certain locations (Settings/Taxes) then the tax won't apply.
Let me know what you find out!
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog