Royal Mail has tightened up restrictions on sending samples through the post.
We recommend that you use a suitable courier service to send the oil samples as this ensures prompt delivery and assists in tracking samples. If, however, you do have to use a Royal Mail service it is our understanding that you can send up to 400ml of sample (e.g. 4 100ml bottles or around 12 of the little 30ml bottles we use) provided the following measures are taken:
The samples are well packed and an external overspill bag is used
The package is labelled with specific information as can be found here
A Material Safety Datasheet is included. If you are not able to source one from the manufacturer of the oil to be tested then you may be able to use this in some of the cases.