Hi Eranda,

There isn't an option to post all of A/P / AR transactions at the same time. However, you could link the posting tasks together using Groups of tasks. Then submit the group of tasks to process or set it as a recurring task to run per a predetermined schedule. Per the online help:

A group of tasks is a list of batch tasks that are :

• launched at the same time (either by submission of the group or by recurring task)
• executed sequentially in time (each task is executed once the previous task is complete) with a log file created for each.
• linked on the condition that the previous task is completed without error.

Their definition is carried out simply by placing a list of tasks in a grid.

Yes it is available 

Go to A/P- A/R Accounting > Invoicing > Customer BP Invoice posting