From Fasttech's website...
List of products that are returned/rejected more often than others:
1 - Lithium battery cells (e.g. 18650, 18500, 18350, CR123A, 14500, 10440 lithium batteries, both primary and rechargeable).
2 - Flammable liquids such as e-liquids, perfume, and essential oils.
Note that since these products are classified as "dangerous goods" by the Post Union, they are sometimes rejected by air mail service providers such as Hong Kong Post. Post FastTech do pay attention and pack these products with special care, however, product packaging cannot and does not alter how these products are classified by the Post Union.
In addition, despite the Post Union classification, each postal carrier has their own rules and practices. For example, Singapore Post is officially okay with e-liquids. For this reason, FastTech implements automatic packaging routing that redirects packages to various carriers according to your shipping country and content of orders. This automatic routing is applied when you select the "Registered Air Mail" shipping option during checkout.
Last but not least, while most packages do get through all the hops and properly delivers to customers, of you are looking for 100% guaranteed delivery time for lithium batteries and liquid products (i.e. you cannot accept delays caused by rejects and/or inspections), FastTech is able to ship through DHL via their Dangerous Goods shipping service. A custom shipping quote is required as shipping costs depend on the destination country, and quantity/type/brand of items ordered (which affects DHL packaging requirements).