It leaked in transit

Thin liquids are the hard case

Diffuser bases are low viscosity by design, because they have to climb a reed. The same property gets them past a liner that would hold a thicker product, so a seal that works for lotion proves nothing here.

The liner is doing the sealing

Not the thread. Check what the liner is made of against the blend, and test the assembled closure, because an incompatible liner fails by swelling or dissolving rather than by coming loose.

Everything ships on its side

Which puts liquid against the closure for the whole journey. Test by laying a filled bottle down for a week before deciding the pack is sound, and then test it again after a month in contact.

Glass breaks, and this is all glass

Dividers so bottles cannot knock together, enough fill that nothing moves, and a carton rated for the weight. Small bottles are deceptively easy to underpack because each one feels trivial.

An oil leak ruins its neighbors

It wicks into labels and cartons and the smell is persistent, so one failure usually means replacing the whole case. That changes the arithmetic on what a transit cap and a divider are worth.

Ship yourself a case

Lying down, by the carrier you actually use, and open it carefully. One carton answers the question that no amount of upright bench testing can.

The pack is part of the product here

A gift-category item that arrives stained or smelling of its own contents has failed before it is opened, whatever the bottle cost. Dividers, fill level and a transit closure are cheap against one replaced case, and at these volumes a case is not many bottles.

References