The TARE settings in CONFIGURATION>>>SHIPPING are taking into account the fact that the packaging you use also has WEIGHT and should be factored into your shipping costs. a product weighing 1kg, when packed, will have a weight of more than 1kg - because the packaging materials will have some weight themselves.
When you uploaded your products, you would have set each product's weight - a ream of paper = 1.00kg, for example
If you are packaging your products in solid gold boxes before sending them out, these gold boxes will weigh quite a bit, so you have to apply a formula that (as reasonably as possible) ADDS the weight of the gold packaging boxes to the weight of the product itself. (If the gold boxes weigh 2kg, and the product weighs 1kg, then the overall SHIPPING weight of the parcel is 3kg)
SEE THE WIKI about this at: http://www.zen-cart.com/wiki/index.php/Admin_-_Configuration_-_Shipping/Packaging
When you have inserted the TARE formulas, you will now need to set your ZONE shipping charges, so in
ADMIN>>>MODULES>>>SHIPPING>>>ZONE RATES
You will set up the countries for each of the zones you choose, and then apply formulas to establish weight thresholds, and the applicable charges.
Your ZONE 1 COUNTRIES could be:-
GB
Your ZONE 1 SHIPPING TABLE could look like this:-
5:0.50,10:1.50,15:2.00
This says
"For packages that weigh UP TO 5kg, the cost is £0.50" (5:0.50,)
"For packages that weigh UP TO 10kg, the cost is £1.50" (10:1.50,)
"For packages that weigh UP TO 15kg, the cost is £2.00" (15:2.00)