This is a bug that has been in stamps.com version 18 and now it is in version 19.
Some valid addresses cannot be selected in a drop down menu.

I have detailed this bug several times with screen shots and several addresses and sent it to stamps.com support.
All I get back is a generic email saying they are sorry, why don't you call in.
I called in once and I get someone sitting at home that has no idea how to fix this.
Hopefully this video will show clearly that this is a real bug.