In act 5, the rational world of Athens and the whimsical fairy world of the woods come together as Oberon, Titania, and the fairies attend Theseus's wedding festivities.
Near the very end of the play might be the best place to put the infomercial for the love potion, when everyone is relaxed. I would have Oberon advertise this rather than Puck. Puck has a darker view of love and life, while Oberon's is joyous, and he describes his entourage "tripplingly" following him. Oberon also has credibility as king of the fairies, an important position. This gives him a positive ethos—especially to the fairies to whom he is appealing.
Oberon might decide to play on the joy of the wedding festivities as a feeling that could be preserved and enhanced with the help of his love potions. The love potions could bring some of the merriment of the woods into this city if the fairies would buy them and use them on the Athenians—this could be great fun. He could describe how simple it would be to administer the potions and perhaps note the pure, organic ingredients they use—nothing artificial. He could mention how love potions could help fathers to get their children to fall in love with the right person, thus reducing intergenerational conflict in Athens. By buying and administering these love potions, the fairies could help the people of Athens become both happy and well ordered! How much better could it get? And the price perhaps isn't so high for these potions.
These are just some ideas. I hope they can get you started as you think more about this subject.