It seems inconsistent. Despite booking on aa.com, some trips I only see 'print trip' on the left side bar. On others, I see option to 'print trip and receipt'. I don't understand why this is, but I guess in those cases where you don't see the receipt you have to call or send a twitter (X) DM to ask them to email you a receipt. Or, as others mentioned, it would be in your original email confirmation. Sometimes I'll just search my email for the trip PNR and find it pretty easily that way.