basic question , tracking sales

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Hi, how do i track with i.e. Binom ( or any other tracker) when people buy e-books on my store ? im attaching last page before payment ... I assume i have to add some pixel there but can anyone provide a bit more detail than that ? I must also note that a placed order witha cc requested auth doesnt equal a finalized sales as there's a % of declined orders so ideal would be to track