Good online marketplace in Kuwait?

It appears you already covered all the options. There is PostaPlus forwarding service that works like Aramex Shop & Ship. They are a bit cheaper, but still expensive for heavy items.

There is another thing though, depending on the item and the seller.

For more expensive or heavy items it may be worth ordering from UK Amazon. If the seller ships to Kuwait (some of them do), Value Added Tax will be deducted from the item price, and that amount can be higher than the cost of shipping to Kuwait, making the total item price lower than the advertised one.

You may want to check if the item you are looking for is available in one of the neighboring GCC countries, and then go and pick it up yourself.

BTW you may want to make sure fitting a moped kit on a bicycle is legal here.