I attempted to make the shopping list work like most online shopping carts work. You specify the quantity and provide the price and it will calculate the total by multiplying the quantity by the price per item.
The issue with that is, firstly I don't think most users know the quantity they are going to want prior to going to different vendors to see where they are going to get the best deal at the time (for the various sized products) and secondly they won't know what the price is going to be until, once again they are at the site and see what the product is selling for, at which time there isn't a need to start adding in those figures into the calculator... your already on the vendors site doing it.
If on the other hand the Quantity area allowed for the entering in of the volume that was being purchased and the cost area allowed for the price that is getting purchased for that item was used, I would think it would be able to be functional... of course maybe it is functional already but it seems as if your doubling what your already doing... placing a 1 in the Quantity and the cost of that 1 is pushed over to the total.
Something that might take a bit of work but would be nice is:
The drop down in the shopping list would allow for the different vendors prices to be shown in the fields. i.e. if I had placed a number of items on the shopping list and now wanted to do comparison pricing, I would use the drop down for say Bullcity next to the item (lets say CAP Sweet Strawberry)... in the quantity area I would put in 30ml and in the cost area (after going to their site and looking at it put in 5.30 (the cost for the 30ml of product) I would do that for each of the items I was planning on purchasing. Then I would go to another vendor like One Stop and again start going down my proposed list with One Stop selected in the dropdown... and be able to insert their pricing in the areas, then be able to toggle back and forth between the two to see which had a better deal for me.
A different use for the area may be that items placed on the list area populated in the quantity and price from what had been added from the last purchase (or most likely from what the user has used in the ingredient section of the calculator where you go in and place the price and amount to add into your inventory on the ingredient listing screen. So if you normally (or had previously) purchase that Sweet Strawberry from Bullcity at such and such price that area in the shopping list is already populated with that info.