How could Lavan claim, "Lo Ye'aseh Kein bi'Mekomeinu..."? Did he not know that when he made the agreement with Yaakov?
Ramban, Ohr ha'Chayim #1: He claimed that, although he personally, was willing to bypass the local custom, the people will (Seforno - did) not let him. 1
Malbim: Refer to 29:22:151:1.
Tosfos ha'Shalem (1, citing R. Efrayim): Lavan said that he assumed that Yaakov knew the custom, and intended to marry Leah first.
Rosh (to 29:18): They agreed about "Bitecha ha'Ketanah." Lavan said that this is Leah (he now called her Rachel), who is shorter (Yefas To'ar is stature).
Tosfos ha'Shalem (to 30:9 (3), citing R. Efrayim): Lavan said "I intended to give Leah to Esav, the Bechor. Now I learned that he sold the Bechorah to you, so it was proper to give Leah to you."
Ohr ha'Chayim #2: He said, I agreed to give Rachel, according to the custom, i.e. not before Leah marries. When you asked for "Ishti," I thought that you meant Leah, according to the custom!
Ha'amek Davar: Since I cannot give to Rachel before Leah, it was not guile to give to you Leah. Malbim - this was good for you. I would not trust someone else to marry her now and work seven years afterwards. I will let you marry Rachel now, and work for her afterwards!
This is why Rashi says that "Nitenah" (29:27) is plural (Tosfos ha'Shalem to 29:27 (1), citing R. Efrayim; and Gur Aryeh - refer to 29:27:2.1:2).
If one was Mekadesh "your daughter" and did not specify which, we do not say that surely, the father gave the older (Kidushin 52a)!
It seems that "we do not give the younger before the older" is not a universal custom. 1 (See Tosfos ibid. DH v'Hilchesa.)
However, the Rashbam (to Bava Basra 120a DH l'Halan) explains that presumably, Benos Tzelafchad married in order of their age, and the Bach and Shach (YD 244) bring this l'Halachah.
Why was Yaakov not concerned for the custom?
Ohr ha'Chayim: He held that a father is allowed to override it. When I asked for Ishti, you knew that I meant Rachel; you should have said that the custom impedes, and I may choose to take Leah, or to wait for her to marry someone else!
