They'll put them on naughty list that will be enforced by financial institutions, i.e. it will be an infraction for credit card operators to process such a payment. Financial operators have well oiled compliance facilities and the payment won't clear. If Temu won't get the money, they won't ship the parcel. And if they won't ship, then there will be a bit less carcinogens in EU. Good stuff.
Just in a poxy country like the UK it's millions of parcels a day delivered across multiple ports mostly inside containers.... It's simply not feasible to check it all, it would cost a ton of money to have enough checkers and not slow down deliveries.
That's a failure of the state to control its borders.
> it would cost a ton of money
That's why the EU is imposing a €3 fixed customs duty per item (and later another €2 handling fee) effective July 1 (should have been much sooner in my opinion) for small packages (under €150), in addition to the VAT.