Let's take the case of the marbles.
Some semi-precious marbles are up to $8 a piece. But I found some internet site that can sell ordinary marbles at 2,90¤/kg (1 kg = 250 marbles of 16 mm of diameter). To make it short, it is less than 1 cent per marble, so I can neglect the cost of the material (well, not really because I will have to pay the delivery at my place and then the delivery to Shapeways, but it is still some cents, say 10 cents per marble).
The most expensive of the pendant (including my markup) is now $10. The less expensive is $6.50. Say, I will stay in this range.
So, the question is how much are the customers ready to pay to get 2 marbles, for objects in this price range...
I would say a couple of dollars.
So in my very special case I would find fair to pay (well, more exactly, I would find fair for the customer to pay
) $1.80 for the service.
The marbles do not have to be inserted into the pendant: this can be done by the final costomer.
The minimum salary in France is less than 10¤/hour, say $30 with the taxes to be conservative.
So if someone paid $30/hr can add 2 marbles in a package in less than 3 minutes and 36 seconds, it can be convenient.