Yes, this seems rather counter (contre?) intuitive, doesn’t it? 🙂

The semantic link you’re missing is that of reciprocity. An action earns a reaction. A delivery earns a confirmation.

In the WordReference entry, the closest definition to the one used here is the fourth: "for, in exchange for":

Je donnerais dix ans de ma vie contre une bonne santé.

The idea is that of an exchange; "contre", in this usage has the following meaning.

(TLFi) III.− Marque l’échange, le rapport de deux grandeurs.
Échanger un sac de billes contre un couteau; parier à dix contre un

In English this meaning could be rendered by "in return for". The word does not make explicit that the signature is absolutely necessary.