Operators from other countries will often use the QSL Bureau system to get cards to the US (or wherever).
Each state call district has a volunteer organization coordinated under the ARRL's direction to handle incoming QSL cards.
Anyone who is (or has been) active on HF should find out what their district's organization wants for incoming cards. Some want SASEs (typically 6x9 envelopes), some prefer funds. Regardless, unless they have something from you, there will likely be some incoming cards that you never get. And some volunteer will probably end up filing the cards for months or years.