Certain items are refused for international shipping from virtually every country because aviation rules, not local law, drive the list. Loose lithium batteries and power banks top it; devices with the battery installed are sometimes accepted with restrictions, but spare batteries by air mail are widely banned. Aerosols, perfumes, nail polish and most alcohol fall under flammable liquid rules.
Biosecurity drives another cluster: plants, seeds, soil, fresh food, meat and dairy are prohibited or permit-only into most destinations, and homemade food is a common seizure. Medicines are restricted both outbound and inbound; a personal quantity with prescription may pass, but controlled substances need permits or are simply banned.
Critically, you must satisfy both ends: the origin postal service's prohibited list and the destination country's import rules. Both are published on the respective postal and customs websites. When a courier clerk asks what is inside, answer honestly; misdeclared dangerous goods carry serious penalties precisely because they endanger aircraft.