Q:

Use Case Modelling

I have 2 questions regarding use cases, 
1. If I signed in to a website and did no action for 30 minutes, the website will sign me out.
How can express I this within the use case model? is it a special requirement for sign in use case?
2. I have a use case called "post a job" which done by an employer. The employer can post one job then proceeds to payment, or post more than three jobs and gets 10% discount of the total amount. 
Which option should I include in the basic flow and which one should I include in the alternative flow?
Here is my attempt:
Basic flow:
....

System displays an option whether the employer wants to post another job or proceed to payment.
The employer selects “proceed to payment” option.
System displays total amount and ask the user to press “confirm”.
The employer confirms the amount.
Make payment use case is performed.
System displays a message that job is posted successfully and direct to main employer page.

Alternate flow:
Step 5: If user selects “post another job”, steps 3 to 5 are repeated till the employer chooses to pay.
Step 7: If more than 3 jobs are submitted, the system apply 10% discount on the total amount.

A:

Sorry, with the sign-out thing you are modeling something with use cases which use cases are absolutely not meant for.
Use cases are present for giving a high level overview of the business values of a system. What you are trying to do is to model a whole business process inside use cases. That is bound to fail. Your use case "sign out" will not reveal any business value.
For the same reason, there is no such thing as a "sign in use case".
"post a job offer", in contrast, is a very good use case.
For the details of "post a job offer", consider drawing an activity diagram. That will visualize the flow and thus make it way more intuitive.
The basic flow is always the shortest and most simple flow, where everything goes fine.

