Might as well put them through their paces and test them (not that I expect your daughter has two fridges fitted with time-lapse cameras!)
I think if they fail within the QVC usual 6 month cut off period the you should be able to return them for a full refund (incl postage) as "faulty" if your plums still wrinkle! Afterall they clearly state on air that they will extend the life of your veggies; so if they don't they must be faulty! QED!
CS would argue you should have returned them under the 30 day mbg but that's more for changing your mind. Ofcourse there's a risk QVC won't accept this, so return them within 30 days, having tested them, and return them as "faulty" and you should get your p&p back and you should get a pre-paid label!
I despair when I read reviews where the buyer says "rubbish threw it away as it didn't work, too much postage to bother returning" Why, when QVC make grandiose claims about these products do folk not challenge them and insist they must be faulty if they don't deliver the results promised on air? Always get your P&P back for faulty items even if you have to ask.
Preaching over. :mysmilie_1: