You do need to enter the weight (put 99g for anything under 100g) and check that the correct weight is showing on the label.
The Post Office cannot accept items with a weight on the label which is lower than the weight of the actual package, even if you have paid the correct amount.
If this is rejected at the sorting office for the same reason your buyer will have to pay to receive their item.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)