I’ll start with the disclaimer. This is most likely an error on [email protected]’s part and orders may be cancelled, but right now 2520 Ninjago Battle Arena is marked at 50% off, on sale for $29.98. However, if you add it to your bag the price comes down an additional $12.50, down to $12.48.
3648 Police Chase from the City theme seems to be suffering from the same additional price off. The product page shows a sale price of $19.98, but add it to your bag and the price drops down to $12.48.
Other sets may be affected but in my brief searchings I was unsuccessful in finding anything.
Thanks to VHTF for writing it in.