This question (and variations thereof) have been asked a thousand times in the osCommerce forum…

I set nup my shipping rates to charge $1 per kilogram. My product weighs 2kgs, and it's charging $5. Why?

For starters, forget Kilograms or pounds/ounces etc. Weights in osCommerce are measured un "units" – and 1 unit can be anything you like. Pounds, ounces, gram, kilograms, magic beans…ok, so once you got that, read on…

The answer is TARE.

Tare weight, sometimes called unladen weight, is the weight of an empty vehicle or container.

Read more at Wikipedia. In osCommerce Tare is automatically set up to 3 (units of weight) when the shop is first installed. It's one of those settings that most Store Owners know nothing about, and is always, repeat ALWAYS the cause of questions that should never be asked…

How to change the TARE

Enter the admin section of your shop, and go configuration > shipping/packaging – you'll find this:

Play with the three settings "Enter the maximum weight", "Package Tare Weight" and "Larger packages % increase".

I usually have those three settings as; 100, 0, 0 in that order. Which means the largest packet I will post is 100kgs, and this is worked out by the usual shipping modules. I add on zero units of weight for postage and packing costs, and I add on zero% charge for larger packages.

When should you use the Tare feature/setting?

If you sell things that always use the same size packaging, eg, a cardboard box, then you could simply insert the weight of the box in the Tare setting. Any weight that is Tare weight, will simply be added to the total weight of actual products ordered.

Of course, using the Tare weight setting also rather depends upon using a system of shipping based around weight! No point setting this up only to then base your shipping prices on dollar value of purchase (for example!).