Hi,
1. There are several solutions to that.
Of course, the standard solution is to use HikaMarket Multivendor:
www.hikashop.com/extensions/hikamarket-multi-vendor.html
It allows you to have vendors, link products with vendors, and of course, the vendors will be notified of the sales of their products.
What's great with that solution for you is that each vendor will only see the sales of his products, so if someone order stuff from different vendors, each one will be notified and only for his own products and won't know about the products of the other vendors.
Otherwise, you can add a custom product field in order to enter the "code" in each product and then display the code in the emails with a simple tag that you can add to the emails template: {LINEVAR:product.field} where field is the column name of your product field. This solution will require the Business edition.
2. It's hard to give you a proper answer without knowing how you configured your shipping method. Could you do a screenshot of its settings ? Also, is your USPS shipper account activated for the use of web services (you might want to contact USPS support about that) ? Did you enter a weight/dimensions in your products ?
3. The version you'll want will depend on what you need to do. So hard for me to answer. Guest checkout is available in both the Essential and the Business editions. On the Business edition product page (
www.hikashop.com/extensions/hika-business.html
) you'll get a list of all the features that the Business edition has and which the Essential edition doesn't have.