There is not so much difference between 払う and 支払う. You don't have to worry or care about it so much.
The only slight difference between them is when you say 支払う, it might be obligatory that you officially pay these fees like tuition, rent, salary, tax, penalty for speeding and so on.
Most of the case, there is no such differences.
When you use it as noun, we say 支払い/お支払い but not 払い。
Some other words(nouns) for お支払い could be お会計、会計、お愛想(おあいそ)、勘定 、お勘定 、精算 、ご精算、チェック。
がんばって!!