When did the Flip employee go to the police? If it was after Corrine had handed the receipt for the parka jacket to the police then the alert has little to no value.
If memory serves, CM did not willingly hand over the receipt. Rather, M Lindsay was waiting on her returning because she suspected something was amiss and didn't trust her/them; it was her instinct telling her to be there as soon as they came in the door. I could be wrong, though, and maybe she was alerted to the jacket by Flip's C Proctor. Regardless, I don't think CM gave any information regarding the Parka of her own volition -- for she was, initially, imo, not going to deny LM had a parka, pretending that the one purchased from Flip on 08.07.03 was the one he always had. The crux here is that this new jacket, crucially, did not have any incriminating DNA on it (which would work considerably in the Mitchells' favour, since it would be impossible to escape from the locus without a single solitary trace of blood on such a large jacket; that really would have dumbfounded the police). If the police subsequently asked (which they obviously would have), for example, why was LM spotted with Jodi at Easthouses at 1655 and then spotted alone at N'battle rd at 1740, then they would have denied it was LM. At this stage, they would be banking on the police believing them on the count that those parkas were very popular jackets and that it was plausible that the person spotted at those 2 places was not LM (still quite a risky strategy, though). Even though the issue of the Parka is a bit of a quandary, the main thing for the Mitchells was that the old parka was destroyed because it no doubt had traces of Jodi's blood on it (probably traces that were not discernible unless inspected up close); outwith direct evidence of LM being caught on cctv murdering Jodi, her blood on his jacket was the next big thing for the police, and no doubt would really have sealed his fate. The Mitchells knew this. Imo, the Mitchells were covering all angles just to be safe, and then taking action and covering their tracks in accordance with what evidence the police may garner (a game of cat and mouse, if you like).