Clean up get() calls, remove commented out code

@Ignat99 PTAL.

As suggested by @dionyziz at https://github.com/OpenBazaar/OpenBazaar/pull/658#issuecomment-56653243 I created a PR out of my own code-review comments.
Actually, if the ```item_price``` or the ```shipping_price``` aren't found, do we want to set them to ```0```?  Shouldn't we log the error and skip the contract?