SKU is more acurate.
Continue is unneccery.
Did some reformatting and added some comments.
Since we no longer need product Id's anywhere just store the object
productquantity needs to be hooked to maketingchannel, otherwise we'll never know how many products got orderd.

If the product does excists we only need to check the prices stored already and update these and check if this product got sold before over the given period and add the amount which was purchased before and update the quantity.