English divides nouns into countable and uncountable. Uncontable nouns are those like sand and rain, where you can't say three sands or three rains but have to say three grains of sand or three drops of rain.
You can only use amount for uncountable nouns. You can only use number for countable nouns. You can use quantity for either.
I have a large amount of sand.
I have
a large quantity of sand.
I have a large number of chairs.
I
have a large quantity of chairs.
So if your products can be both uncountable and countable nouns, you should use quantity. However, quantity sounds wrong if you're talking about money.
I see no advantage to using amount rather than total for the total cost. You can get away with it when you're talking about shares, since shares are countable, so amount cannot refer to shares. However, if you also have uncountable nouns possible for your products, then people could get the amount column and the quantity column confused. So I'd recommend using your second example.
Your current account balance is $X smaller/less than is required
It is possible, at least hypothetically, for a "balance" to be negative. A large negative balance is certainly not smaller than a small positive balance, but it is less, by the accepted meaning of the word.
To say that one amount is smaller than another is, strictly speaking, to preclude the possibility of its being negative, or otherwise to assume that one is speaking of non-negative amounts. The word "balance" implies an amount that could, at least hypothetically, be positive, negative, or zero---as balances tend to be explicitly signed quantities, debit or credit in some sense---and therefore to me it does not seem appropriate to say that one "balance" is smaller than another unless one is speaking in terms of absolute value, but even this would not usually be inferred on its own without explicitly making the situation clear.
For example, if A's balance is -$5.00 and B's balance is -$3.00, then both of the following would be true:
A's balance is less than B's.
B has a smaller negative balance than A.
In the original example, without further context, I would only use less than.
Best Answer
I need a small amount of milk in this recipe (generally implies a single dose). Same application for Ireland's small amount (viewed as a total amount).
I need to add small amounts of milk (probably need to add gradually, while cooking). Ireland's small amounts of waste were being promptly disposed of throughout the year.