At no time have I argued that petty theft is not immoral. It is, under the principle that theft itself is immoral; violating that principle is a necessary and sufficient condition for being immoral.
Gilligan’s example is a very good for illustrating a situation in which the total harm done is distinctly less than would be the case were the same amount of money directly removed from someone’s bank account. Not knowing that you were to receive the money is not what accounts for there being less harm; it’s that one’s bank account remains the same as without it, rather than being emptied. Analyse it as game theory: if you had to choose, would you rather not receive money owed (say, double the amount you currently have), or have some amount stolen from you (say, half of the amount you currently have)?
Where knowing vs. not knowing that you were harmed matters is that, insofar as harm done to you is unnoticeable because it has no practical impact, that harm is theoretical, at best. Remove a penny from the bowl on my desk when I’m not looking, and even if I notice you do it, I probably wouldn’t say anything. Steal the rent money in the desk drawer while I’m in the bathroom, and I’ll feel very wronged.
Rather than provide you with a ready-made straw man, I’ll quit using metaphorical terms like “moral calculus”; in return, you stop trying to trap me with the argument of the heap.
I wouldn’t notice if a dollar disappeared from my bank account, and it would have no practical impact on my life; I’m certain that at least some others are the same. If less than every victim notices the harm (and thus suffers no harm at all, in practical terms), then the total harm done is less than the case where the money is stolen in a lump sum (i.e., the case where the entire disappearance is felt).
If you think that argument depends upon an objective, unit based measurement of harm, remember that the sentence “grey is less dark than black” makes perfect sense without resorting to a light meter.
Email me your address, and I’ll write you a cheque.