What do I pay at checkout

Two things: the trial fee, and the refundable deposit.

Nothing else is taken at checkout. You are not paying for the item at this point, and you are under no obligation to buy it.

On a £200 item that is a £16 trial fee and a £20 deposit, so £36 in total. On a £500 item it is a £30 trial fee and a £50 deposit, so £80.