"Le valigie sono pronte" is correct. "Le valigie sono pronti" is not correct.
"pronto" is an adjective in this case, and therefore declines accordingly to the noun it refers to, even if it is before the verb.
"Il pranzo è pronto."
"La minestra è pronta."
"I panini sono pronti."
"Le valigie sono pronte."
I hope that helped. :)
Frustrating yes but consider the structure of the sentence. In "Ho gia` le valigie pronte." the person/owner is already established ( ho =I have) therefore they are "mine". I agree that it is better to include the possessive (le MIE valigie) to make things clear rather than relying on a convention but apparently that is Italian.
In this sentence, said by the tour guide to the bus driver, the suitcases are the subject not the person so the bags could belong to any one .
I wrote 'he cases are ready' This was quite obviously a typo. I obviously didn't press hard enough on the T. Sometimes DL accepts genuine mistakes as typos so it is annoying when they don't accept what seems so obviously a typo. I don't suppose it was the 'cases' they were objecting to as we probably use the word more often than 'suitcases'