Your giftee is counting on you and they are waiting! If there is no way to avoid late shipping, please do not mark your gift shipped. Instead, send your giftee a message from the retrieve match page and let them know that they have not been forgotten.
If you do ship late, Reddit Gifts will send you several reminder emails. If you are very late, your giftee will need to make a no gift report to be credited for the exchange, despite you having contacted them about the issue. This will result in you being banned from redditgift exchanges until you prove that you have shipped a gift. Proof of shipping can be submitted directly from the exchange page and any issues on your account will be resolved once your proof of shipping has been hand reviewed and accepted by a human.