Now let's try get your arithmetic right.
You say you're working in KILO ...
But you're talking of a "letter" weighing 100 GRAMS
Your zone shipping numbers are:-
100:0.50,200:2.50,500:3.00
Which means:
"For weight up to 100 Kilograms, charge 50p"
"For weight up to 200 Kilograms, charge £2.50"
"For weight up to 500 Kilograms, charge £3"
(500 kilograms is half a ton - find me a shipper who will ship half a ton for £3 !)
You also need to look at the numbers you are giving to each product. Again, are you working in GRAMS, or KILOGRAMS. I don't know because I can't see the numbers you've applied to your products.
You must choose a weight unit and apply it's arithmetic principle throughout your website.
So if you choose KILOGRAM, and you want to express weight in grams (or fractions of a kilogram, then your numeration must be "1 gram = 0.001"
So your 100 gram letter should be 0.1, (not 100 as you have stated in your zone shipping).
I am sure that if you correct your numeration throughout the site, we will make a bit more progress on this issue.