At least for me, the online store worked well last Thursday. No error messages at any point, even though I spent a lot of time fiddling with the selections. There was another club night last week, and there can be traffic spikes during those, which makes error messages more likely.
The price increase in the shopping cart after the discount code is apparently because a double-discount block was recently added to the system, meaning promotional items lose their discount percentages if you try to finagle newsletter discount percentages into the same order.