only one prod in shop - ticket and discounting like free for child

[pefa]

hello,

 

I am planning Prestashop with only one product.

 

Because I will selling only reservations I have speecific needsy:

 

I have 60 pcs of tickets and I would like to give 1 free ticket to 1 paid - it would be promotion for parrtenst with childs. When someone buy one ticket should could 'buy' next (one) ticket without fees as adding next item to the basket.

 

I know that buyers could write annotation in additional informations but I have to keep limits - I have specific quantity of possibile persons-buyers

 

 

As it is important to me please let me know all your experiences with similar problems. thank you !