[Shasown]

Who is to say she was murdered in or even near her flat.

Supposing she brought the cider and pizza in order to go to someones else house or flat for a bit of a session.

She goes home gets showered and changed, while doing so empties her purse or pockets and in amongst that stuff is the receipt for the pizza, after getting changed she then takes the cider and the pizza round to the party location.

She may not even have taken the pizza to her flat having already dropped it off at the other location.

Incidentally, the barcode on the pizza box will simply tell plod that the pizza was priced at and made by.... Its the batch/lot numbering with the best before date etc on the box that will indicate the box came from the same lots that she may have bought one of.