analysis - Point of extension - Use case -

suppose have use case buy book, , main flow following:

1- user types book code wants buy 2- system replies there's enough stock of requested book 3- user confirm 

very simple.

now suppose want give option user thing between 2 , 3. how should it? guess it's extension use case, i'm not sure it's point of extension.

as far know, if choose, say, point of extension in 3, user has opportunity 3 or extension not 3. same behaviour of alternative flows.

but want different. want "2.5" or nothing... or nothing instead; not thing.

i'm sorry vague question.

one option format recommended in alistair cockburn's writing effective use cases:

2a-  user wants thing: 2a1- user thing 2a2- system responds in way, returns step 3 

step 2a occurs after step 2 , before step 3. if uc ends @ step 2a2 replace 'returns step 3' 'use case ends' or similar.

hth.