Visa appointment about invitation letter

In my invitation letter,the date of arrival and departure is 27th April - 27 May and i am planning to come to embassy monday 30th April,Is the invitation letter still valid?
If i can not schedule visa appointment on line,is it at the embassy at any time i come to the embassy and how?
Will i pay the 14000 naira visa fee before comming or when the approve my application?