In this chapter there is a dialogue I have a couple of questions about. This is the end of the dialogue:

Продващица: Вы купаете шапку для себя?
Алексей: Нет, для сына.
П: По-моему, ему больше понравится чёрная.
А: Хорошо, дайте черную. Я её куплю.
П: Пожалуйста. Заплатите в кассу.

There is an e, not an ё in the last черную. Is that a typo is there a grammatical reason for it?

Also why is кассу is the accusative and not the prepositional case? I would think she is saying "Pay at the cash register" not "to the cash register". Or is this idiomatic?

Thanks,
Z