You can close the online store whenever you wish and you don;t need to inform anybody.
Howver if you then choose to close the comapny that operated the store, you go about that in the normal way.
If you don't owe anything to revenue or to suppliers, then its fairly simple for an accountant to do, however if you do owe money to anyone you either need to pay it off, negotiate the debt or put the company into liquidation.
If there are no assets left in the company, then you simply cease trading and inform creditorsa nd revenue that you have ceased trading and that if they wish, they can appoint a liquidator and that you will keep all files relating to the company for a period of 6 years should they wish to inspect them.