Just to reinforce SuKi's ver good answer, she is correct about US usage. We use "mail" for all these uses relating to physical letters. "Post" is never used in our country in this way.

I'll presume you are talking about sending physical letters and parcels and so on, rather than the digital meanings. The verb to 'post', in this sense, is the equivalent of the US verb to 'mail'. If you post something, you put a stamp on it and put it in a letter box, or you hand it over the counter at a post office. By doing this, you are consigning it the official postal services. 'Send' can mean to 'post', but it also has a more general sense. You can send a person somewhere. You can send an oral message, such as best wishes, via a person. You can send a parcel by giving it to someone and telling them to give it to someone else. In British English, 'the post' can refer to the official postal service. For example 'Don't forget to put this letter into the post.'. It can also by a specific aspect of that service e.g. First Class Post. It can also refer to the letters etc themselves. 'What time does the post come?' 'Did you get any post today?'. Most of these noun uses of 'post' also correspond to the US term 'mail'.