Addressing Envelopes in Spanish?

You had it right! It is perfectly fine like that

Are those Spanish-speaking households in the US? If yes, use English on the outer envelope because you can't expect everyone at USPS to read Spanish.

If not, use Spanish.

I agree with @Nancy, the USPS probably doesn't know how to read Spanish all that well. I'd address them in English and then make the actual invitation Spanish.

@Nancy that's a really good point. Maybe don't address them in Spanish but have the stuff inside in Spanish?

Does the addressee matter if the address itself is in English? I think Spanish is fine for the names, and yes those are the correct abbreviations.

Thanks for the input, yall! I didn't even think about whether the Spanish speaking households were in the US or abroad! (We have some of both, actually.)

Many other cultures have doubl barrelled last names as well. Make sure you know how each couple prefers to be addressed!

I'm not sure if being able to read "Sr. y Sra." is the hardest part of delivering the mail. So long as they can read the address, who it's addressed to shouldn't be an issue?

It really does not. all USPS needs to know is the address, which is understood in both languages