Lost, I'm not sure how it works in the UK, but here, yes, the USPS (and other shipping companies) will do that. I assume you don't get mail on Sundays, but it may arrive tomorrow. It's happened to me multiple times (and always seems to be on things I really need or am excited to get, not boring, non-urgent things). If you don't receive it within the next couple days, I'd check in with the place you ordered from.