Sending a parcel from the UK to USA?

So I bought stuff today that I'm sending to my friend in California. It's two 500ml cans of irn bru, bar of galaxy, some scottish tablet, shortbread, and a few other sweets, also a few British coins

I can't weigh it myself because I don't have scales, also I don't know what box I'm getting yet because I still need to get one. But I really don't want to spend over £20 sending it. Anyone got any idea of how much it might be, also will the tins of irn bru and coins be allowed?
Also will I just need to put the stuff in the box, write her name and address on it and then take it to the post office and tell them I'm sending it to America? What do I have to do exactly?

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