For some reason in the game the mail isn't always delivered to the mailbox, I'm not sure why, but it happens. And when its not delivered to the mailbox you can usually find it somewhere around the mailbox. So go into buy mode and find the bill, then move the bill a few spaces away, then see if your sim can access the bill. It seems that when its delivered on the ground there can be like an invisible barrier that makes the bill not able to be accessed, but if you move it a few spaces it goes away. I'm not sure if it will work for you, but it seems to work for me. I hope that helps.