Relevant information like "I can look up the price but you won't be able to purchase this as the tills close at 6?" you mean?
If it's before 18:00 and the sales assistant has assumed the transaction will be completed before then, the time the tills close isn't relevant in that moment. Perhaps there was time for the transaction to be done but by the time the assistant had explained the pricing issue and the OP had decided they did want it, the system had ticked over to 18:00 and it was no longer possible.
The excuses you give could all be valid, but they are just that - excuses for poor service.
It's not poor service, if the customer comes in "just before closing" no matter what the assistant does they cannot physically do anything once the clock hits 18:00 regardless of how soon they told the customer about the till system.
Also, shop staff are still human, we make mistakes, we forget things, we're sometimes too busy dealing with the customer's pricing issue and dealing with that query to notice that it's 17:58 and by the time you've explained the issue between the two prices and whether or not the store can price match the online offer price that it will be too late to process a transaction.
If, as you suggest the assistant gave "the best customer service the assistant could offer under the circumstances" why the need for the long list of excuses?
They aren't "excuses", I was merely trying to illustrate that there are many reasons why the assistant might not have explicitly stated the fact the tills went off at 18:00 on the dot.
However, once the tills were locked, the assistant could not do anything more, even if they wanted to. At that point they could have just said "Too bad, so sad, we're closed so come back some other time" but they didn't. They apologised, explained the issue with the tills and then went to check with a more senior colleague that there was absolutely nothing they could do to make this purchase happen then and there. Once it was confirmed that there was no way to override the system, what was the assistant supposed to do?
I am confused how the odd 10-15 minutes here and there adds up to 2/3 hours a week
Because for every day when I "only" get out of work five minutes late, there's one like last night when I clocked out almost an hour late because a customer couldn't accept that the shop was shut and there was no way we could put their purchases through and kicked off until we had to threaten a call to the police.