Why is there an extra 'Shin' in Yisachar?
Rashbam: It denotes a double reward, one for giving Rachel the Duda'im, 1 the other, for offering her maidservant to Yaakov.
Ha'amek Davar (to 30:17): It was her reward, and also Yaakov's; 2 he consented to marry Zilpah, who desired him. This is why it says "l'Yaakov" (30:17).
Why was this a reward for Yaakov? In any case he would have 12 sons, and he most desired sons from Rachel! Perhaps he was destined to have at most one or two sons from Rachel; he preferred sons from Leah over those from his other wives (Rashi to 33:1). (PF)
Why does Leah attribute her son to giving her Shifchah? Rachel did so, and did not have a son [yet]!
Riva: Rachel did so for she had no sons. Leah already had sons.
Malbim: Rachel did so, for she relied on Segulos; 1 it did not help her. I do not rely on Segulos; the episode of the Duda'im shows this. I gave my Shifchah for her benefit, therefore Hashem gave my reward!
Presumably, the Malbim means that Rachel gave her Shifchah thinking that it will help her, just like it helped Sarah.
Riva, Da'as Zekenim, Hadar Zekenim #1: One Sin is because Leah bought the right to lie with Yaakov, and this is dishonorable. 1