The answer is in this lesson. This is a case of the direct object pronoun preceding the verb - agreement of the past participle with the direct object pronoun is required. It is a more advanced level lesson so you likely have not come across it yet.

In this case < les > is the direct object pronoun, plural (and feminine, because it replaces < ces chaussures > ). Therefore the past participle < voulu > takes the terminal < -es > in agreement. 

Special cases when the past participle agrees (in number & gender) when used with 'avoir' in the compound past in French (Le Passé Composé)