tall bearded

What if you don't offer more that one option? Only standard post and no ability to select a registered option at checkout?

The choice of the delivery service to be used rests with the buyer, and the legislation clearly imposes an onus on the seller to do as buyer instructs. That is, if in my listings I only provide for regular post, and the buyer says no I want it sent registered, then the seller must do as the buyer instructs, because if they don’t, they may well find themselves at risk until the item is actually received.