Let's see the definitions (NOAD):
Quantity: the amount or number of a material or immaterial thing not usually estimated by spatial measurement:
EX: the quantity and quality of the fruit can be controlled.
EX: note down the sizes, colors, and quantities that you require.
Amount: a quantity of something, typically the total of a thing or things in number, size, value, or extent.
But for amount the second meaning given is "a sum of money",
EX: They have spent a colossal amount rebuilding the stadium.
Amount can also be a verb, unlike quantity, and it's the following:
Verb [intrans.] (amount to) come to be (the total) when added together :
losses amounted to over 10 million dollars. [And others side meanings not related with "quantity acception".]
I'd like to note that the two are given as synonyms by the Oxford American Writer's Thesaurus, but it's pretty much obvious that they're not absolute synonyms and therefore are interchangeable in some contexts only.
So considering all of this, if you mean the total of products, I'd use quantity, if instead you mean the total of money, I'd use amount.