Hi, this is quite an urgent question.

We have a store which sells jewelry and watches. Watches have a set retail price, but gold prices change daily depending on the gold commodity price (xe.com will give you that). We don't want to change the prices on ALL the gold products every day. We want to change the "exchange rate" once and have all prices changed automatically.

One huge bonus is we are not plugged into an online payment gateway like Authorize.net, rather we just use the plain old CC module and then we process payments manually with our point of sale merchant machine.

One idea is to set up watch prices in US$. Like eg. Rolex XYZ = $5000. And then set up a fake currency called GG (Grams of Gold) where we have Small Ring = 20 GG, Bracelet = 15.5 GG. Then we use the currency rate to edit the value of GG. So let's say we discover the price of gold is $10/gram today then our prices are:
Rolex XYZ = $5000
Small Ring = 20*10 = $200
Bracelet = 15.5*10 = $155

and then tomorrow the price of gold is $20/gram we have:
Rolex XYZ = $5000
Small Ring = 20*10 = $400
Bracelet = 15.5*10 = $310

Note that the customer should NEVER see the value quoted is GG, it should ONLY show US$. And when the payment gets mailed through to us it must come as $US not GG. This is important because the GG value changes every day so we need to know what the resulting price that the buyer saw on their screen.

So just to sum up:
1. the value of GG comes from us manually entering the currency value once per day.
2. we set up prices once in a made up currency called GG and from there actual $US prices are quoted on to the buyer
3. the web site should enforce that $US is the "only" currency and shouldn't give you a choice of paying in GG (as this makes little sense).

I know that with the flexibility of Zen Cart and the experience that you guys have there will be an elegant solution shortly.

Thanks,
Paul