The main thing about shipping is to FIRST establish WHO you are going to use to do all your shipping. This can be one organisation, or several organisations, and may include private courier services, and/or the postal services (such as Royal Mail).
Once you have selected your shipper, you need to get your head around HOW they charge, and HOW they calculate these charges. So, a comprehensive RATE CARD should be obtained from them.
In the case of Royal Mail... things get a little more complicated, because RM has almost 3,000 different parameters for posting stuff around the world. There are different rates for different countries, and of course a huge array of services (signed-for, large-letter, first class, second class, insured, small packet, large packet, next day, economy... the list is almost endless).
In any event, you need to make a choice about WHO you will use, and get a good understanding of how their services are configured.
NEXT, you need to look at how you can incorporate these configurations into zencart, and again, different modules provide different parameters.
If you are UK-based, and want to use RM / ParcelForce, then your zencart site should be properly configured for UK use. Standard ZC is configured "generically", so you will see that things like date formatting is in the USA style of mm/dd/yyyy and that spellings are USA (color vs colour, check vs cheque) and that weight parameters are LBS and not KG. Also, there are no UK Counties loaded in a standard install, and as many RM services are based on UK locality, your website needs to have these configurations built BEFORE you apply a shipping module.
Getting your shipping accurate is critical for TWO reasons:
- Too cheap, and you will fund the difference out of your bottom-line profits.
- Too expensive, and people will abandon your site and go elsewhere.
Once you KNOW exactly who you wish to use for your shipping, and WHERE in the world you are prepared to sell to (or deliver to), come back and tell us and we can make some more pointed suggestions.