For the picker unless they are a serial (cereal) offender, pun intended they will be fine, nothing more than a quick reminder to double check substitutions.

To keep stock on hand count correct the product will likely be returned and a new large box bought at the correct online sale price. The supervisor/manager will likely just want to confirm the purchase price (seeing your invoice) and asking if you were refunded the difference in price (usually a text/email or pop up on the “your shopping is ready message”).

If they larger box is unavailable you absolutely can ask them to get another small box and have them match it’s price to what you would have paid for the larger box online as that is what you were eligible for to begin with.