If you have a FAAS form that has accidentally gotten 2 quantity for the same product fields in it (which I know is wrong, but accidents still happen), the Post Processor Concatenates those fields causing massive overcharges to appear.

For example:

<input name="Quantity1" id="Quantity1" value="1" type="hidden" />

And then the client later on adds this somewhere else on the form

<input name="Quantity1" id="Quantity1" value="1" type="hidden" />
Then the Transaction gets processed as:

<Quantity>11</Quantity>

It would be better to either, ignore subsequent values, or return an error about multiple quantities being passed.
Pretty much, doing anything other than combining them would be a better solution than surprising a donor with with an exponentially larger withdrawal from their credit card.