Shipping paintings from Spain

Hi everyone! I’m new to DPW. I know there is already a topic about selling from France but my case is a little bit different…
My paintings are 30x30 cm on 1,2 cm plywood so I think I can’t sell them as a letter. The price is little higher than 100 $. so my questions are…

  1. Which are the cheapest options to sell in the US?
    2)Do I have to pay customs taxes?
  2. Do I have to fill any form?

In general, can anyone share their experience of shipping from Spain with me? Thank you!