"Simulate payment" is what a customer would do on the checkout; here it is called from this shop's server through the API, the same way your own test setup would.
What the shop received
2026-09-21T08:06:29.478Z · invoice.created · livemode false · signature ok · status created
2026-09-21T09:06:38.871Z · invoice.expired · livemode false · signature ok · status expired