“Hi (ex-roommate), you’ve seem to have left (insert items here) behind when you moved out, as well as taken things that I’d bought. If you don’t want the things you left behind anymore that’s fine, but if you want any of it text me by Wednesday at the latest to arrange a pickup. Otherwise I will assume those items are abandoned and do with them as I see fit. You can return my items at anytime by leaving them in the mailbox.”