Learn Spanish (Latin America): No obstante, la correspondencia está lista y no tengo con qué comprar el sello. - However, the correspondence is ready and I have nothing to buy the stamp with.

This is a negative existential structure, which is the focus of this lesson. It uses no tener (to not have) + preposition (con) + question word (qué) + infinitive (comprar). It literally means "I don't have with what to buy." This structure is very common and natural in Spanish to express that you lack the means or tools to do something, replacing heavier phrases like no tengo nada con que comprar.

Why does qué have an accent mark here if the sentence is not a direct question?

In these specific structures (like no tener con qué or no hay de qué), the word qué functions as an embedded interrogative pronoun, so it keeps its written accent. Think of it as containing an unspoken question: "With what can I buy it? I don't have [that thing]."

Is no obstante just another way to say sin embargo?

Exactly. no obstante means "however" or "nevertheless" and is used almost identically to sin embargo. It sounds slightly more formal, which fits well with a formal-sounding word like correspondencia (correspondence or mail).

A quick refresher: why is it está lista and not es lista?