Update price

The `update_price` API allows for scheduling price updates for a particular site. By specifying an `effective_date`, future updates to prices can occur. If price updates are made in real-time, the `effective_date` can be the current time. The API takes both a debit/cash price (retail price) and a credit price for each product. If only a single price is available, use the `retail_price_per_unit` field. The API may be called as many times as prices change during the day, or in a set interval. Additional pricing data such as tax information and contract details between merchants will be provided separately (via email, ftp, etc). **Update Behavior:** The `effective_date` parameter governs whether this API will do an insert or a replacement. If there is an existing update with the same `effective_date`, the API will discard the original data and replace it with this request's data. Updates with `effective_date` in the past will be rejected with a 400 Bad Request. There is a small tolerance allowing for clock skew. Real-time updates should use the current time to ensure they are effective.