A criminal is abusing you for money laundering.
The $1800 you received are stolen. You are being used to launder these funds. The $1200 are another account controlled by the scammer. By funneling them through your account, you are making it harder for law enforcement to trace them. Same with the iPhones. They are either bought with stolen money or stolen themselves. By asking you to forward them through the mail, you are breaking the paper trail which allows law enforcement to trace their ownership. They will soon be sold on eBay or by a street vendor and thus be converted to clean money the scammer can use freely.
When the person those $1800 were stolen from reports the fraud, the bank will reverse that transfer. But the $1200 transfer will not be reverted. You will be out of $1200.