Rashi (Bereishis 30:10) says that Zilpah was younger than Bilhah. Why did Lavan give the younger maidservants to Le'ah, his older daughter (and vice-versa)?
Rashi (later on 30:10): He did that in support of his trickery, so that Yakov should believe that he was marrying Rachel (and not Le'ah). 1
Since it was customary to give the older maidservant to the older daughter and the younger maidservant to the younger one.
Since it says that he gave Zilpah "l'Leah", there was no reason to write "Lah"!
Hadar Zekenim: He made Yakov think that he was giving her to Rachel. 1
However, it says "Lah" superfluously also when he gave Bilhah to Rachel (verse 29)! This is difficult.
Here it says that he gave Zilpah "Shifchah". It says (verse 29) that he gave Bilhah "l'Shifchah"!
Tosfos ha'Shalem (4): Zilpah was very young, and not so capable of serving.