Even good companies make mistakes. If they were receptive to you telling them and addressed it with you then I'd be likely to give them a pass. Only time this would differ is if I knew the company had a history of sloppy privacy controls etc. Most likely this is a human being that was stuffing envelopes and when they got to your letter they accidentally folded the other companies letter and sent it to you unintentionally (or it was given to them that way).
My personal view, people often expect perfection until it is their own mistake, I wish people would give others (including companies) the same benefit of the doubt they'd like to receive when they make a mistake. Not saying you aren't but assuming they at least apologized and said they have addressed it I don't think there is any reason to do anything else. Of course, if you know this company is sloppy, first why would you use them, but second then it is more fair to raise this issue more publicly. But unless you want your every mistake judged the harshest way possible, I say accept people (who work in companies) will make honest mistakes, address it with them and move on unless it is chronic or abusive.