While all of what you said is theoretically true, it is a far less likely explanation than the seller simply pocketing the difference between the advertised postage cost (and indeed the one paid by the buyer) and the lower price which it actually cost.
We don't KNOW exactly the situation but Ockham's Razor is rarely wrong. By the way Ockham was a 13th Century English philosopher who postulated that the simplest explanation was usually the right one - and given we're dealing with horny, greedy, petty, deceitful creatures (aka "humans") driven by venal, cheapskating, profiteering lusts and a total lack of moral fibre, I'd say we're looking at a simple case of the seller knowingly taking full advantage of the opportunity to rinse the postage - rather than your esoteric and defensive alternative. Unless you KNOW your less likely explanation to be the case, why on Earth would you take the time and trouble to propose it when all it does otherwise - most particularly if you're a regular seller yourself -is raise questions about your own ethics?
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