Usually merchants will be charged every time they receive payment by credit card, and therefore when you pay by credit card the price increases. But when paying with Bitcoin, merchants will not be charged any fees because the payment will automatically go to their wallet, so it is natural that you pay less than you should because they are not charged any fees when receiving payments from bitcoin.
I believe the expectation is exactly the opposite because I rarely see a price reduction when choosing Bitcoin instead of Fiat. At least in my case on the bigger websites. Or the merchants may use more expensive prices. Newegg may be doing this, allowing them a larger margin to subtract from in order to attract Bitcoiners. After all, it has been many years since I last heard about a big merchant starting to accept Bitcoin so the opportunity is still large.
OP, I have an idea. Print the Transaction image and put it some where like in a drawer you only check every few years or a time capsule. It will be a very interesting thing to find in a matter of years after you forget about it.