“You were supposed to mail that package last week. Now it’s going to be late.” Bringing up a mistake just for your own satisfaction of hearing them admit they were wrong doesn’t resolve the situation.

Blaming, complaining or nagging causes the other person to feel like a failure. No wonder that they defend themselves. They may call you out for nagging, excuse/ justify their action or refuse to respond altogether. None are what you wanted.
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There’s a better way to say it…

Communicate with intention. Say what you really want. “About that package…..” Do you want them to know how you feel? Share how their inaction or action impacted you. “When it wasn’t sent, I feel that what is important to me is not important to you.” Do you want to create a plan to avoid repeating the situation in the future?
Create a plan together. “Let’s look at our system for ensuring that packages get out on time. What can we do better?” Do you want to remind them to take care of it? Ask with sensitivity and dignity. “It is important that the package gets out by tomorrow. Will you be able to drop it off?” –
And keep in mind – everyone is human and messes up from time to time. Let go of the expectation of 100% perfection. “Regardless of the mistake, I still love you anyway.”