I do not know about American English, but in British English 'goods' and 'merchandise' are more or less interchangeable with each other but not with 'commodities'. So you could say something like:

<em>A jewellery shop keeps a lot of its goods/merchandise in the shop window.</em>

but to say:

<em>A jewellery shop keeps a lot of its commodities in the shop window.</em>

would not sound right to British ears.

'Commodities' has several meanings, the most common in modern usage (in my experience) signifies tradeable raw materials such as oil, gold, foodstuffs (sugar, coffee etc).

None. Those are all synonyms.

 Different words with the same or similar meanings.

   Goods can be contrasted with Services, which are the opposite, consisting of actions and services performed by   people for pay.