My son's pants

I went to alter my son's school trousers. Both were the same size. When i went to collect them my son went in as i wasn't well. I didn't check because I trusted them . Recently when my son had to change the pants i found out that the other pants are way too big to be his. Today I went there to return the pants and hoping to get my son's right size . They checked and told me they can't find the size that I'm looking for and told me to bring in the other pants so that they can identify the pants . I told them that I am coming back . I rushed home to get the pants , gues what when I come back five minutes later they were all gone, doors closed . I went to the door to knock but there was no one. They should have at least told me they were closing and should come back the following day . Yes i will go there to sort out this , but I am very upset