I don't think it's a matter of weight, I think it's a matter of clearing customs.
If you order A, B, and C from a vendor the packing slip/invoice shows A, B, and C. If (when) customs opens up the box they find bottles of A, B, and C and all is right with the world. If instead they open up the box and find bottles of A, B, C, and X, things start to look shady. When things look shady to customs packages get delayed and customers get unhappy.
This could be avoided by having an actual orderable item for "sample" at a cost of $0.00, so the item shows up on the invoice. The tricky part would be to have the logic built into the shopping cart to only fulfill "sample" when the order total is greater than $15 after discounts and before shipping.
Another option would be to have folks add the item "sample" to their cart for the cost of $4.99 and then apply a coupon code like "freesample" (NOT A REAL CODE, DO NOT USE!!) to the order to apply a $4.99 credit to the order. Then the tricky part would be allowing the use of 2 coupon codes (1 for the free sample and 1 for the 10% off or free shipping) on a single order.