You have to include your HST number on any receipts, invoices or contracts with more than $30 of taxable goods. If you attach the slip to an invoice that has the HST number you're fine, but if the slip is the only thing you give to your customers and you charge more than $30 it *has* to have an HST number on it.
I always attach slips to invoices with an HST number on it, so it's a non-issue for me.
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