It all depends what market you operate in.
If you're in highly competitive low-margin retail sector (selling exactly same goods as your competitors) sharing your savings with customers could give you a crucial advantage.
But if you're an artist, selling your paintings with large profit margin, giving 1-2% discount wouldn't matter that much. But then again, there's also the PR factor, if you choose to share, you'll be perceived as more genuine and honest merchant and you surely would get more sympathy from bitcoiners.
As for your oil price drop examples. I think the most important reason for that is what goose20 wrote:
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2. Consumer expectations - we get accustomed to a certain price level - no consumer calculates what input costs are; and
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Also, some businesses (such as UPS/FedEx) simply cannot be too flexible with their pricing. Adjusting prices every week or two would be seen as very unstable and unpredictable by their partners. I'm sure when the opposite happens (oil price goes up) they won't rush with increasing their charges.