I am sure there will be foster children that would want to write to people they keep in touch with. This is called pen palling. I even do it myself, although – not really successful at it. While I have a PMB, there are some strange rules where they will charge extra for additional names using the mail box. With that said, I will acquire a PO Box, and depending on where I live will depend if it is financially viable to keep the PMB. If not, I will keep it for one more year, and change my addresses to the PO Box. The child will receive a manual on these things including the PO Box number. This PO Box will be acquired no matter what, so it is possible for me to have 3 addresses.
No matter how good of handwriting/print the foster child has, I will recommend that they use the printer to print their address in the return address location, and the destination. This means a postal carrier will have no issue in delivering their mail. There will be a laser printer in the bedroom, and there will be a priority document feeder for the instances to use the envelope. They will then affix a postage stamp on the envelope, and set it in the outgoing mail wall pouch where I will drop the letter off when I go out again (usually within a day or 2). They are responsible for their postage stamp, but keep in mind that they will receive money for doing chores.
They will not be able to use the mailing address when they leave my custody. In which case, unless they know the address of where they are moving to, they may have to rely on email even if temporary. I would have helped set the child up with an email address (or 2). I personally provide my email address on my letterhead of my letters which is printed on the page immediately. The correspondent will have all options available to them.