Normally, using the FedEx, USPS or UPS services is easy if you want to connect to their API and website to get accurate shipping cost for each service. You would enable the Zencart module and configure what services you want to offer. The cart contacts the service, gives the weight, etc and comes back with a list of services and prices.
Using USPS is cheap and convenient for us, but creates a number of issue. We use Flat Rate (if it fits, it ships) service. A cardboard envelope might cost $5 to ship anywhere in the U.S. but the cart can't automatically determine what can fit in an envelope. The same holds true for the Flat Rate boxes.
So we fudge things. We generally ship similar products in different package forms. Thus my 1 pound bag might be a different shape than my neighbors similar product.
I created a spreadsheet for each neighbor, using their most common items ordered and information about how they normally ship. Thus they may find that they can ship two one-pound bags in an envelope, 5 bags in a medium box and 8 bags in a large box. The spreadsheet then creates a list of prices to be entered into the shipping module (like in the Zone configuration).
The cart allows you to enter any number for a product weight. Because most shipping prices seem to be commonly in pounds, we fill in the item weight in pounds too. (1/2 pound would be .5). Thus if the customer buys two 1 pound bags, the cart adds the weights together and does the lookup in the US zone and finds the price for the envelope and charges that.
For the other 2 zone configurations, we go to the second (International) and change the country abbreviations to only include countries we really expect to ship to. The supplied ones are not necessarily useful. We then check USPS shipping costs to those countries. In general we just check UK and use that price as many of the international countries are the same price.
We then check Canada shipping price and configure that for the third zone.
The only gotcha in this is if you forget to include a country code and someone whats to ship to there.
One other gotcha is that in our case we have one product (unroasted coffee) that is twice as heavy as roasted coffee. This means that 2 pounds of unroasted coffee fits in the same space as 1 pound of roasted coffee. So in that same envelope that 2 pounds of roasted fits in, we can fit 4 pounds of this product. We fudge things by turning off displaying "weight" to the customer. We then go to the unroasted products and say that they weigh .5 pounds (half a pound) instead of a pound. So hen a customer order 4 of these, the cart thinks it is only 2 pounds and will fit in that one envelope.
Although this sounds a bit weird, it does work for us to ship a few different product sizes and be able to cover a weird exception. In the case where we just can't get the weight to package stuff to work, we include a reasonable shipping cost into the product and bill it as "Free Shipping" and say it will ship for free if purchased separately or with other items. Lets say the actual shipping cost is $1. Assume your order history is that 1/2 order this product alone and 1/2 order with something else. Raise the product price by 50 cents. 1/2 the people will pay an extra 50 cents over cost and 1/2 get a 50 cent better deal. On average you will get your postage paid. Alternatively, if most people buy the item by itself, add a dollar to the price and mark as free to ship. In the corner case where someone buys the item with another item, throw in something free to make up for the hidden dollar they paid fro shipping that product that you put in a box they are also paying shipping for.
Remember, there is no such thing as FREE SHIPPING. It only appears that way because the cost has been added somewhere by someone.
