Coins as collectibles can only be sent with parcel / package post - i.e. the more expensive way, they are classed as goods, and are insured at a higher value and tracked more by the postal network if sent via a postal service other than a parcel company (obviously the parcel company also takes greater care with tracking and pays out more when lost or stolen).
Money in this sense is not illegal to send, only money laundering regulations make large cash amount illegal (and lets face it if you are sending large amounts of cash in the post you are doing something illegal), don’t forget large amounts of some de-monetised notes can still be exchanged in national banks, etc.
So, the reason Post services will not allow coins to be sent via normal post, same rules applies to small DIY items, etc - for example screws, is because 1; they are worth a value and will often get stolen (so they want you to send via their packet post service) and 2; letters are sorted by machines which cost millions and sort millions of letters per day at very fast speeds, these letters whizz through slots to have the postcodes read and are redirected at various points, with the machines constantly moving etc. Idiots who think someone is going to sort their mail by hand all the way to its destination are living in the past, these machines are very delicate and are designed to process paper, the coins, screws etc are ripped out by the machines and cause stoppages, damage and delays.
The price of post is regulated by certain treaties, and is designed to help developing countries have cheaper prices than that of post being sent from developed countries. If you are in a developed country post to other countries can be expensive and visa versa, these treaties are updated slowly so some countries that have been considered developed for many years might still have post prices from 30-40 years ago when they were considered developing. When this changes the international postage can go up 4 fold overnight and can come as quite a shock to people from that country, but that is just the way it is.
So your cheapest way to send anything is via your national postal services, because as said their prices are regulated by international treaty, parcel services have more scope to add costs to these services - mostly on the promise of better service and faster delivery. Local postal services will also offer package services which cost more because of the extra insurance (to mitigate theft) and because of size (so they are tracked more and not sorted through the big machines for post). Local postal services will also offer tracked mail which will be placed in the machines if the size is excepted, this will be cheaper than their package service, but will have caveats such as no goods, coins, screws etc! Because it is tracked it will have a small amount of insurance on it (you can pay for more) check with your post office how much they will pay if it is lost or stolen. If you place small amounts of coins, packaged so they do not move around and the envelope is regular in size like documentation and not totally rigid they will not break the sorting machine and you will have no problems. When declaring the contents you have to lie (because if the postal services rules so no coins or goods you will invalidate your chance to recover costs if it is lost or stolen) so declare „collectible items“ to the correct value or the maximum value of refund from the postal service.
Now for customs, if you declare „collectible items“ and customs spot it be prepared to pay some fees (usually the recipient pays these) here you need to know if the country charges Value Added Tax on coins or not (well the recipient does as they will be paying and or disputing), customs or the local authorities will not bring criminal case against you or the recipient unless you don’t pay the tax, and then only for not paying and not the importation of the goods.
Hope this helps.
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