Hi Claudia,

The rules of agreement of the past participles with the auxiliary 'avoir', you will find in the following Kwiziq lesson -

https://french.kwiziq.com/revision/grammar/special-cases-when-the-past-participle-agrees-in-number-and-gender-when-used-with-avoir-in-le-passe-compose-conversational-past

So, it is 

Je les ai reçues ( les=les lettres) 

But 

J'ai reçu les lettres 

The verb 'recevoir' is not a verb of state but here are a few -

https://french.kwiziq.com/learn/theme/1512981

Bonne Continuation!