I sent out mail last month to another user here in Canada, but they never received the mail. I've had no trouble sending to others, and this person has a fair amount of good feedback.

We've been messaging back and forth, but haven't come to a conclusion on what to do.

What would be the best way to proceed? It was a fair amount of value, and I haven't received any cards in return (I was to send first). Do I open a bad trade report?

According to Deckbox rules:

2.1 After the trade is accepted and addresses exchanged, there is no backing out. The transaction is considered confirmed. If a user needs to back out because he cannot complete (for objective reasons, like missing cards) the trade, he must be willing to negotiate a compensatory trade with the other party.

2.2. Sending cards

The sender is responsible for getting the cards to the destination
The sender will be held accountable for items lost in the mail if a form of tracking was not used. Proof of sending is not sufficient
Senders cannot be held responsible for loss of packages after delivery has been proven.
If a problem has occurred, please resend your part of the deal, return the cards you received (if applicable), or make appropriate monetary restitution (the parties involved can decide on which one of these options is best)

In this case, because you were required to send first and your trade partner has not sent his cards yet, there are a few options. Because neither of you have received any cards - if you both agree to it and no negative feedbacks are requested - this trade may be cancelled (See rule 2.1). Another option is to re-send the cards you have sent, should you have multiple copies. Another option is to reimburse your trade partner with cash/paypal. The choice is up to you and your trade partner. Both parties must come to an agreement.

Not sure if Canada has it, but in the US there is a certified mail option where there is a confirmation from the post office when it goes through and confirms that it was sent, that tends to be the best option in the US for sending and a Canadian version would probably be a good bet as well for future trades.