What is a Group of postel letters called?

There isn't a specific, commonly used term for a group of postal letters.

Here are some options you could use, depending on the context:

General:

* Batch of letters

* Collection of letters

* Pile of letters

* Set of letters

More specific:

* Mailbag (if they are all together in a mailbag)

* Correspondence (if they are letters written for a particular purpose)

* Mail (a general term for anything delivered by postal service)

Ultimately, the best term to use will depend on the specific situation.