This scenario feels like the best fit to me - explains why she returns home and then goes out again, after finding a missed delivery card on the doormat (if it was a shopping errand she could have picked it up en route - either at the mini market on Melrosegate or by asking to be dropped by the Costcutter).
The timing out and back to East Parade PO fits. The apparent presence of her passport in her handbag - used as photo ID when collecting the parcel. Also, possibly the presence of her handbag in the bedroom - CL having gone straight upstairs on her return to try on the new clothing. On that basis you might suppose that the order was unlikely to be a hat, scarf or coat.
The obvious question would then be, was the wrapping or contents of any such parcel found in the house? Presumably this would be known to NYP as a result of contacting any catalogue companies she had dealings with and routine canvassing of local shops and businesses in 2009.