bikfoto, there are three kinds of product photography:
1. Drop & Shoot: your standard place-on-table, light and shoot job. Assembly-line style, if you will. One product after another, lighting doesn't really change, you just make minor adjustments when you change the subject. Think of a bottle of perfume on a light table: record shot. No frills.
2. Beauty shot: still life mechanics. You have to arrange the subject so as to stop, even stun, the viewer. Props might or might not be needed. Creative arrangements are the order of the day. Think of same bottle of perfume resting on a rock with a forest or beach as background, light shining from above giving it an aethereal quality.
3. Illustrative photographs: where you're selling the need for the product. The product is usually not even in the photo (although the designer might drop it in as a superimposed layer in the final ad). Think of a beautiful woman in an autumn afternoon, leaning on a railing with a yearning look in her eyes.
As you can imagine, type 1 is the cheapest and type 3 the most expensive. Which kind are you doing?
High volume photography is of the first kind, and unless your client has deep pockets, you'll be looking at charging in the tens of dollars. Adjust as per the number of images, and charge a set up fee per set to be built. Say, $50 as set-up fee and $30 for 5 shots, then start lowering as the number of products increase, say $27 for up to 10, $23 for up to 20, $20 for more than 20. This is just an example: if you're in a metro area you can even charge $100 per product; but in a small city, if you go past $50 per pic they'll just find someone else.
$15 is too low, BTW, unless you're in a hamlet.
If you're doing type 2, then the whole game changes, as it can take you several hours to construct the set and get the shot. These are the jobs for which you charge a day rate.