I'm going to buck the trend and expose myself as the bounder I am and suggest you should base your decision on your impression of the company.
If it's independent and you've otherwise experienced good service, do the right thing. If it's a big chain / someone who has a ropey reputation, leave the 2nd delivery boxed up for a month and if you get no contact you've got some free spares
Tell them. I don't hold out too much hope it will go well.
My experience is that they either expect you to do all the work, including paying up front for postage which, given their incompetence so far, may leave you out of pocket. OR They'll promise to send out 'a label' or arrange collection.
Over the years I've experienced all of the above. So much so, that if it costs less than a £10 note, I don't even bother.
That's disappointing - personally if a company expected me to pay to rectify their mistake while I'm holding all the cards I'd tell them to jog on.
Reminds me of a situation some years ago... ate at a chain restaurant with the mrs and when the bill came half the stuff was missing. Being the idealistic, morally-correct idiot I am / was I let the staff know of their mistake. The clearly disinterested, rude 12 yr old serving us actually seemed resentful of the effort required to revise the bill and I left thoroughly angry at myself for going out of my way to do the right thing.
In an independent I'd do the same again, but never again in a large chain.. and I'll never eat at Byron Burger either as the quality of their staff has been consistently pitiful IMO (probably due to cack training and wages, so IMO they deserve the repercussions).. but I digress!
Good luck in finding an agreeable resolution to your moral dilemma!