How are you automating shipping rates for e-commerce sites?

We are in the process of developing an e-commerce site for our wholesale plumbing/waterworks/irrigation site, and are running into a snag trying to automate the shipping rates. How has everyone handled this process, when a shipment can contain small fittings that can be combined into 1 box, but also potential for large items like bathtubs and waterworks pipe. We don't want to gouge our customers, but we also don't want to lose money on online orders. I put we are using Zen Cart v151, but we have not, as of yet, decided if Zen Cart is going to work for our needs.

I've seen many different approaches... Everything from the extreme of including the approximate shipping costs in the product cost (and using free shipping)... All the way to the other extreme of sending detailed product (and packaging data / shipping origin information) to 3rd party vendors such as USPS / UPS / FedEX and having the 3rd party provide a shipping quote. There are a number of other methods which fall in between... Such as doing flat rate based, weight based, zone based, or table based shipping estimates...

I'm sure there are other methods as well but those are the most common I've seen.

CGRemakes:

How has everyone handled this process, when a shipment can contain small fittings that can be combined into 1 box, but also potential for large items like bathtubs and waterworks pipe. We don't want to gouge our customers, but we also don't want to lose money on online orders. I put we are using Zen Cart v151, but we have not, as of yet, decided if Zen Cart is going to work for our needs.

If you are located in Australia:
http://www.zen-cart.com/downloads.php?do=file&id=899

This module supports all of your requirements, and a whole lot more.

Cheers
Rod

if you are here in the US, these are the ones i am using for computer stuff

http://www.zen-cart.com/downloads.php?do=file&id=1292 <=USPS

http://www.zen-cart.com/downloads.php?do=file&id=1293 <= UPS

https://www.numinix.com/downloads/zen-cart-modules/shipping/fedex-web-services-shipping?zenid=c3266b277e3cccb9bfc8bb051ca7046d <=FedEx

The USPS and FedEx ones you will end up having to create accounts with them and getting the account put over to the production server instead of the test server to be able to get everything working. all you need to do is call them(worked better) or email them.
Hope this helps. Thanks to the folks here helping me out with this also. They pointed me to what i needed and went from there. Good Luck