You should check out the list of trading rules:
https://deckbox.org/help/trade_rules
and trade guidelines:
https://deckbox.org/help/trade_guidelines
Specifically it mentions:
2.2. Sending cards. The sender is responsible for getting the cards to the destination. He will be held accountable for items lost in the mail if a form of tracking was not used; proof of sending is not sufficient. Tracking should show the package delivered to the proper address.
If you as a sender do not have proof of delivery (or the package was wrongly addressed), resend your part of the deal, make monetary restitution or return the cards you received. Resending cards should always use a form of tracking, to avoid further disputes.
Failure to honor your part of the deal without reimbursement is sanctioned by being marked as a scammer, and your trade partner will file for mail fraud.
Additionally, in case of lost mail, the sender must open a formal case with the postal service (for USPS you can do so by calling 800-ask-usps) and provide a case number to the receiver (and to site administrators, if a dispute is open).
In this case you need to talk to your trade partner to figure out what to do. You could sent back the cards that you got, you could send the cards that you have and some others to make up for the ones that you don't have, or whatever arrangement you can agree to. In the end though, it's your responsibility until your side of the trade is delivered.