Way back in the dim, distant past when I first started learning French, I think I remember being told that to accept an offer, you say "s'il vous plaît" and to decline, you use 'merci".
Eg.: Voulez-vous un verre de vin? Oui, s'il vous plaît./ Non, merci.
I think we also learnt that if you simply replied "merci", it would be understood as declining the offer.
Is this correct?