We had to swap formula before when OH bought the wrong type but it was about ten minutes from getting it home to taking it back and he swapped it at the Asda without trouble.
They are used to him though, he's also taken back the wrong type of nappies on more than one occasion and the wrong sized baby clothes and the wrong wipes (Joseph has sensitive skin and I get the sensitive wipes for him).
There's one checkout woman who comments if he buys anything alone now, asking if he's sure and suggesting he rings me to check before she rings it through!
But they are usually fine and have never argued about anything we've needed to return.