Creates an L<FS::TaxEngine> object. The subclass will be chosen by the
'enable_taxproducts' configuration setting.
-CUST_MAIN and TIME are required. OPTIONS can include "cancel" => 1 to
-indicate that the package is being billed on cancellation.
+CUST_MAIN and TIME are required. OPTIONS can include:
+
+"cancel" => 1 to indicate that the package is being billed on cancellation.
+
+"estimate" => 1 to indicate that this calculation is for tax estimation,
+and isn't an actual sale invoice, in case that matters.
=cut