"Your GST # does not have to be on envoices under 30.00 Canadian Even if you charged GST "
That is technically correct.
For transactions under $30, the Act requires
Vendor's business or trading name
Date when GST was paid (or payable)
Total amount paid or payable
For transactions between $30 and $150, the Act requires
Vendor's business or trading name
Date when GST was paid (or payable)
Total amount paid or payable
An indication of which items are subject to GST and either the total amount of GST
Vendor's GST registration number
For transactions over $150, the Act requires
Vendor's business or trading name
Date when GST was paid (or payable)
Total amount paid or payable
An indication of which items are subject to GST and either the total amount of GST
Vendor's GST registration number
Purchaser's name or trading name
Description of item
However, in practice, most registered sellers include their Registration number on all invoices, regardless of value.