I'm pretty sure this would not be a Canadian specific issue.
From our Federal Revenue Department:
Invoicing requirements
Disclosure of tax
s 223
35. The Act imposes certain requirements with respect to the disclosure of the tax on
invoices, receipts or written agreements to provide recipients with sufficient information
to enable them to ascertain whether they have satisfied their liability to pay tax on taxable
supplies. A person who makes a taxable supply must furnish, upon request, sufficient
information, in writing, to substantiate a claim for an input tax credit (ITC) or rebate.
Disclosure of tax
ss 223(1) and (1.1)
36. Registrants who make taxable supplies (other than zero-rated supplies) of goods
and services and are required to collect the tax must disclose to the recipients that the tax
has been charged on the supplies in one of the following three ways:
(a) by indicating on receipts, invoices or in written agreements the consideration paid or
payable by the recipient and the actual amount of tax payable in respect of the supply
in a manner that clearly indicates the amount of tax;
(b) by indicating on receipts, invoices or in written agreements that the amount paid or
payable includes the tax payable in respect of the supply; or
(c) if the registrant chooses to indicate the tax on invoices, receipts or written
agreements, the total amount of tax payable or the total of the rates (7% or 15%) at
which tax is payable must be indicated on the invoices, receipts or agreements.
Page 8 15-1-e.pdf http://www.cra-arc.gc.ca/E/pub/gm/15-1/15-1-e.pdf (opens PDF)
Reference in this publication is made to supplies that are subject to the GST or the HST. Effective July 1, 2010, the HST applies in the participating provinces at the following rates: 13% in Ontario, New Brunswick and Newfoundland and Labrador, 15% in Nova Scotia, and 12% in British Columbia. The GST applies in the rest of Canada at the rate of 5%.
http://www.cra-arc.gc.ca/E/pub/gm/15-1/README.html
When BC goes back to PST and GST in April, the PST rules had a similar requirement but their website is lacking this info at the moment.
It doesn't look like AvaTax would solve my problem, unless I didn't find the right information.