When you agree on a set price for a script and the script writer says he's finished and wants the other %50 cash, is it nornal for him to say: "Ok, I'll install it on your server" rather than giving you the script with installation instructions and then saying it'll be $10 more for instalation instructions?
You should've agreed upon who would perform the installation prior to paying him and having him start the project. If you agreed that he would install it for you to help save you time, and then provide you with instructions to install somewhere else at a later date (in the event that you need to)..then sure it's normal.
In my opinion, if you hadn't talked about it ahead of time, I'd say that it is expected that he would just have YOU install it, however if he is willing to do it for free..then great!
Don't pay him the other 50% prior to him installing it on your server, and once he does, download a copy of it. The last thing you want is to pay him the last 50% and then have him run off, leaving you screwed. If he's reputable at all he'll let you see the deliverables BEFORE requiring that you hand over the last bit of cash.
Again, something you should have talked about before hand. If you agreed he'd write you a tutorial on how to install it, then of course it's normal. Without pre-discussing it.. I'd say "any reasonable person would EXPECT the coder to provide some sort of written documentation for a quick install". Maybe not an in-depth 20 page ebook on how to do it, but having him write you a quick 10 step installation guide, shouldn't be asking too much of him.
If it's really a little php script, as you put it, then it shouldn't require much documentation. If you REALLY need the documentation..well, I guess you'll pay for it, but it shouldn't be much $$ right? I see now that you said he told you afterwards it'd be an extra $10 for a guide. I think that really depends on the quality of his work and the relationship if you have with him. If you really like the work he did, like how he treated you, and he did a good job on the install and is willing to write a few pages for the installation to make it real easy for you next time... pay him the $10 and next time you use him (if you do), tell him ahead of time the quote he gives you better include installation instructions.