There are so many goddamn layers of absurdity to this that I felt compelled to write them all down.
1. Namor chugs an unlabeled bottle that randomly shows up for him in the mail.
2. He does this before reading the note that came with it.
3. Or, evidently, the return address.
4. The person who sent it anticipated not only that he would drink any old thing that showed up in the mail, but that he would drink it before reading the note.
5. The person, knowing this, sent him a potion that would kill him under highly specific circumstances instead of just sending him poison.
6. They also sent a note explaining this so he could avoid it rather than letting him fulfill those circumstances unknowingly and kill himself.
I would also put Namor hanging around on dry land wearing a speedo, but I don't know the character very well and that could just be something he does.