Argos 3 for 2, returning an item.... how much refund do you get?

So the Argos 3 for 2 toys is on at the moment.

What happens if I return one of the items?

For example....
I bought 3 £15 items and paid £30 (3 for 2)
I want to return one, how much do I get back? If I have my receipt do they refund £10 (as I got 3 for £30) what if I dont have receipt? Will they exchange at the current selling price? The item does have the argos product number sticker so there should be no doubt I got it there.

Also....what if I reserve to collect another 3 more items on the 3 for 2, £15 each as before total £30 assuming its before the 3 for 2 ends! And I return one of the previous items, bought previously in the 3 for 2. Do I pay £15??

Hope this makes sense!