I don't know about the notice requirement, but what I did was sent notices via USPS with delivery confirmation.  

I do not know about your area but where I live providing a cluster box is the responsibility of the post office as it is their decision not to make door to door delivery. 

We absolutely support persons with disabilities, however as seen on TV, folks taking advantage of the system are not far from your doorstep.

I would just mention mail / fedex/ UPS delivery was asking at  the house for the sister and ask why.

I always figured if it was delivered with delivery confirmation, without required signature, that was good enough.  

Be sure you send it with delivery confirmation, like USPS letter with delivery confirmation.  

Just send it with delivery confirmation.  

And if you do it that way, rather than get them to sign a new agreement, then I'd send it by priority mail with delivery confirmation. 

If they don't honor it, they are in breach.I would write them a letter, with delivery confirmation (emails work fine, if that's how you've been communicating, as it has a date stamp on it), that says you only want that unit, so they can honor the lease or pay you back twice your holding deposit (or whatever you decide) by end of next week, as a settlement for breach of contract.

As far as email, it's legally considered a "writing," but I think I'd do both - send the email telling them of the rent increase, "and I'll also be sending this notice via USPS priority mail with delivery confirmation."