Your order number is how we connect your payment to your order. Interac e-Transfers do not carry your order details on their own, so the message or memo field is the only place where we can see which order the money is for.
If you forget to add it, your payment is not lost, but it cannot be matched automatically. That means a delay while we sort it out by hand.
If you already sent an e-Transfer without the order number, please open a support ticket in the help center using the Contact support button with your order number and the name and email used to send the transfer, and we will match it manually.