These seemingly nice peoples from the Avinsome have writ to me exclaiming their puzzlement and apologies for the disappearing CD's.
They are making enquiries to the courier company that claim to have delivered and received a signature for said parcel, and have assured me that they'll be back in touch within the next 4 days.
If they can't be traced for whatever reason, they will be taking full responsibilty and arrange for free replacements asap.
Can't actually get much fairer than that imo.