. . . I find it helpful anyway.
After shopping, when I get home, before putting the shopping away, I get out my sharpie pen and the receipt and write the price on each item. If it is something like rice, I also get out my calculator and work out how much per 10g or how much per portion size. If it's not possible to write on the container, I write it on a label and stick it on!
Then I keep the receipt, in case I need to refer to it again.
(another aim is to get the number of receipts down!)